IF the OP really does have a bonafide keylogger trojan, standard virus removal tools won't detect it nor fix it. These things are very, very challenging to find and remove. Unless you know specifically where to look and what to look for, it'll never get found. And once you find it, removing it is another challenge. Can it be done? Yes. Is it something a novice user can reasonably expect to be able to do? Probably not.
These things are resistant to detection AND removal. They are often enterprise grade software. They're not little scripts written by bored teenagers. These things are meant to steal valuable data or monitor usage. Unlike viruses most people think of, where people assume that the virus is meant to cause obvious chaos, these things are MEANT to go undetected. They are designed that way. If they get detected, they can't keep spying on you and stealing things from you.
How can you get a keylogger? Or any bit of nasty malware anymore? Surfing the internet poorly protected- and firewall/antivirus/antispyware is not enough anymore. Downloading stuff isn't required anymore. You can pick up bugs just going to websites or reading emails. Chinese gold-selling sites for MMO games are especially notorious for keyloggers.
Reformatting your harddrive may or may not work. These things are meant to survive that. Most of the nastiest malware WILL survive a reformat. Erasing a disc does not really erase anything. It's very easy to lift data off "erased" drives and you don't need to be in an FBI lab either. Why do you think businesses and the government just warehouse their old drives?
Anyway, the idea of a keylogger trojan is very tinfoil hat. I'm not saying it's impossible, because sure, it is- a couple of my WoW guildmates have been keylogged, and I know for a fact my worklaptop has all kinds of big brother software lurking in the background- but this DOES give her ex an awful lot of credit for being cunning and savvy (and quite a lot scary!)
Working on the assumption she really does have a trojan quietly running in the background, the simple & complete solution is just to replace the harddrive. Replacing the whole computer not necessary. A new drive is a complete, simple and inexpensive solution.
The advice to copy over all her files is pretty bad. IF she has a bonafide bit of yuck, just doing a massive "My Documents" copy may very well pull the source of the bug with her. When doing the backup INDIVIDUALLY choose files ONE BY ONE. Do NOT take whole directories. Things like saved emails are probably not safe. Saved email attachments are not safe. Any kind of demo software or what-not is probably not safe. If you're going to go with the tinfoil hat trojan theory, you have to look at EVERY file on your computer as the possible source of infection.
ETA: If you REALLY have things like saved emails and the like that you feel you will NEED (ie, emails for pending litigation) I'd probably print out copies, and then burn digital copies to a CD. Archive both, making a note that there might be bugs attached to the files. Then archive the CDs and keep the printed out stuff in a safe place for reference. I wouldn't access the contents of the CDs unless I HAD to, and I'd do my best to access the files only on a computer that I didn't care if it caught a bug, and it had no sensitive data on it. My laptop, for example, is pretty good for this- it's old, it barely works, it doesn't wander the internet and I don't use it for anything anymore. If it caught 500 viruses it wouldn't matter, because it's never exposed to any other computers.
Physically destroy the old drive before disposing of it (see note above about nothing erased is ever really erased...)
