Ok, I'm an IT guy (computer nerd) for the government. I'll tell you this:
1. Spybot and AdAware are both bs. Neither of them really work. They both show false positives and as said, "one picks up what the other doesn't"; yeah, that's the false positives.
2. Limewire, and all of those file sharing programs, leave open connections to your computer once you install them and those connections stay open even if you close the program and then turn the machine off and on.
3. The only way to get rid of them, sorry to say, is a wipe and reload. That'll remove all program installs as well as registry keys that will open ports. These ports can allow a hacker to put a virus, worm, or other malicious code on your computer. We can even put a key logger so if you do any online banking or financial transactions, it'll catch all of your keystrokes for your passwords and user names.
4. Just wipe the machine and reload, if you don't know how, the machine should have came with "restore cds". These are what you'll need. If you want to backup (burn / save) any files, make sure that they are ONLY files you created. However, before you drop them back on the wiped out machine (after it's been reloaded), make sure you scan the cd with a virus protection program to make sure there are no viruses, trojans, worms or other malicious code on the cd.
5. Any antivirus program works. It depends on how up-to-date you keep it. If you're not updating it daily, it won't do squat. I prefer Symantec but they are all personal preference, you pick what YOU want, read some reviews on the net, ask some questions at the store, check the benchmarks on the programs (statistics for catching, detecting, removal of viruses and other malicious code). The thing that you may like about Symantec is that you can call them if you can't get rid of a virus and they'll do it for you, charge you like $50.00 though but it'll get rid of it. My sister used it before she called me, she's not bright but oh well.
Let me know if you need help.