Old set-in cat odor problem?

My daughter moved out last year to marry and join the Air Force. She had the world's laziest cat, it would not bother going down the steps to the basement litterboxes, whereas my 16.5 year old cat prefers to be let out, will only consent to "the box" if it is raining or snowing. He'll hold it forever.

Daughter's cat was gotten as a kitten and she daughter never made the effort to reinforce the litterbox idea. Now, that she has left for tech school, I'm soon to be stuck with her cat a month or two, until they find housing. We had been planning on converting her old room to my husband's office (we're both work at home writers) and money to replace the carpet from the cat accidents (I call them "deliberates", lazy cat) wasn't available until now. I need to sequester that cat, along with litterbox and food and water dishes in my daughter's old bedroom & don't want to buy new carpet & pad twice this year. We've just kept that door shut, so the odor hasn't been a problem, out of sight, out of smell, out of mind, but with the cat coming back, I'm afraid she'll hit her old spots and we'll never rid the room of the odor, even with new carpeting and padding after she leaves.

How can I remove old set-in pet odor problems? Specifically cat pee. I'd like to have the room and carpet fresh, so she's not tempted, she'll have her litterbox handy. I have to keep her sequestered, my cat would freak out to see her again. I don't need additional odor problems, I want to clean up the old odors and once she's gone, repaint & do all the carpeting for my husband's office, he really needs that room, he telecommutes and is too cramped in his current location. It can't be nasty and smelly in his new office, it needs to be a pleasant work environment. I'm willing to try shampooing a thousand times, anything to get the set-in smells out, so she's not tempted to use the corner instead of the litterbox and I can piant & recarpet after she leaves, along with moving office equipment and rewiring to accomodate his business needs.

Any advice is greatly, highly, sincerely appreciated and I am waiting in anticipation of a solution to my woes. Cat arrives about mid-July, maybe a little bit earlier.

Thanks again,

deb