Just days after Barbara Bush moved into the White House, the first lady summoned her staff to the living quarters to lay out a simple plan of action. "I want to do something every single day," an aide says she told them. "Let's try to do some good every day - it's time to get serious."

Mrs. Bush might have exceeded her own goal were it not for an unexpected - and welcome - hitch: The volume of mail generated from each of her 40 solo events was so overwhelming, she has said, that at times she felt forced to slow down in order to catch up.In the past 100 days, the first lady has received 35,000 pieces of mail.

If judgments on George Bush's first 100 days are tentative, still reflecting a wait-and-see attitude, appraisals of the other Bush in the White House are hardly reserved. Barbara Bush seems to have swept into the nation's collective consciousness like a "breath of fresh air," a do-gooder, a down-home mama and the professional grandmother she just happens to be.

"Well, I think I'm a fraud," she joked in a telephone interview Monday. "I've always been the same!"

In her first 100 days, she dished out food at a homeless shelter, held the hand of a little boy who said no white lady had ever touched him before, read to minority toddlers at the Library of Congress, kissed an AIDS baby and hugged an AIDS-infected adult, sent for drug czar Bill Bennett so she could offer to help and opened her home to hundreds of disadvantaged people who would underscore her special concerns.

"It's been wonderful," she said Monday. "You can't believe - I have really loved every minute of it. Of course, I have always been one to think that you should love your life."

And for a woman who never much cottoned to the spotlight, her aides says she has become particularly skilled at directing the glow to suit her own objectives.

"I think it has been an opportunity," Mrs. Bush said, adding that she doesn't make a "calculated effort to put the focus on anything."

"I don't know that those things are going to generate attention. (But) I very carefully pick things that need help, and if it helps - that's wonderful for me. I'm not there for self-aggrandizement. I'm there because they need help.

"And don't forget," Mrs. Bush continued, "I don't fool around in the U.S. government. I leave that to other people. This is the way I can help."

Says her press secretary Anna Perez, "What she's done here is figure out how to make lemonade from lemons."

During the phone interview, the first lady dismissed the liabilities of her new job. She said she hasn't felt that her spontaneity has been stifled but conceded that it still was a bit early to judge. "I know it will come," she said with a chuckle.

The first lady was asked if she was unsettled by last week's discussion at the White House (attended by the Bushes and Vice President Dan Quayle) that explored a transfer of power in the event her husband is disabled.

She said she was not.

"We thought it was a very good idea" to have the meeting, she said. "I think it made others uncomfortable. Let me put it this way: I would (have been) very uncomfortable if the Reagans had that meeting with me attending. You don't want to think about it - but it's very important to know the ground rules. You ought to talk about it."

While few of any political stripe are willing to make on-the-record comparisons between them, the contrast between the White House styles of Barbara Bush and Nancy Reagan are apparent almost daily. During Nancy Reagan's first months in the White House, she was tagged as a conspicuous consumer who was more interested in $200,000 china and designer dresses than in social causes. Whether she attended a ladies' tea or an anti-drug event, people often seemed focused on Mrs. Reagan's physical appearance, from her cosmetics to her Adolfos.

By contrast, when Barbara Bush shows up at any number of events, men and women can be overheard applauding her personality.

"She's the big hit of the Bush administration," says Larry Sabato, a political analyst and professor at the University of Virginia. "For one, Democrats see her as a breath of fresh air. She's less star-struck, more sincere - like Betty Ford in some ways. She doesn't talk about herself personally - but she has the same directness. At some point in time, she may stay much more popular than (the president)."

"She really enhances him," says Paul Costello, who worked for Rosalynn Carter in the White House, and was Kitty Dukakis' press aide last year.

Says Phyllis Coelho, wife of House Majority Whip Tony Coelho, D-Calif., "If you close your eyes and listen to her you'd almost think she was a Democrat. You just know she cares about the little people."

If life seems satisfying for the first lady these days, sources close to her say it hasn't all been a rose garden. For one, her recently discovered thyroid condition has taken its toll. While she has insisted that she "never felt better," she has allowed that it may be time for her to start pacing herself.

Last week she told a group of reporters, "I'm lucky if I get through the day. I don't mean I'm overextended. But I'm 63 years old and I need to be babied a bit."

Monday she elaborated: "Nobody asks me to do all these things - I do them because I want to. But I have overscheduled myself. I'm not going to go into retirement. I'm just not going to try to do 12 events in one day. That's silly."

Also, by her own admission, she has had to adjust to unrelenting press attention to her and her family as well as the more structured aspects of her new life.

"She (still) does have this idea that she can do things the way she did them before," says a senior White House official, "and it just can't be."

For the first 100 days, anyway, the first lady has another view. "I do exactly what I want to do," said Barbara Bush.