At times like this I back up my important files, then format the hard drive. There will be better ways of going about it but that's the only easy way I know of. Unless of course your on Windows Vista, I can't tell you how long I've been searching for a way to format my Vista machine. Why is it so hidden?
(That isn't a dig at Vista btw, I seriously can't find out how and would love to know)
First, don't download programs. Not only can it get you in legal trouble, but hackers and other jerks use the more popular ones (like Adobe) as a means to propogate their trojans and other viruses.
Second, get some anti-virus software. I recommend AVG, Avast, or BitDefender. Actually, I recommend anything that isn't Norton, even McAfee.
Third, run said anti-virus software. I wouldn't back up any files until either the trojan is gone or quarantined.
Fourth, buy Adobe Lightroom. It'll be most likely be cheaper if you're a student or have a friend who is a student who buys it for you.
Yeah your best hope is obviously to format the drive. I know we hate to do it. But if its a poorly made trojan, theres nothing to worry about, but if its a good one, well then you won't be able to pierce it with anti-virus, and it could cause significant evils upon thee (hint: don't buy anything with your credit card on that comp). But formatting has a 100% chance of removal. Along with 100% chance of removing everything you ever loved and held dear
To format a drive in Vista, I think you just go to the Control Panel, System Section, go down to Administrative tools, then click Create and format hard drive partitions. Then you can format it from the Action menu.