Any idea how it got onto the computer in the first place? Friend/sibling/roommate used your computer, or you went to new websites, or ??? Here are some deterrents:
1) use a Limited account (aka Restricted User account on Win2000) for browsing/IM/email. Inherently lacks the power to install stuff, even if successfully exploited. Regardless of what exact browser/IM/email program you use, this is a strong limitation on what it can do behind your back.
2) keep Windows/etc patched up (duh).
Microsoft Baseline Security Analyzer is a useful follow-on to a Windows Update session.
3) ensure that all Administrator-class accounts on the computer have a complex password, eg
EvanAdams@AT. This goes for the hidden "native" Admin account too, which MBSA will help you determine if it's weak/blank. If necessary, you can set an account's password using the command
net user username newpassword (may be useful for XP Home users).
Leave an un-password-protected account named "Visitors" that is a Limited account, so your gf/roommate/sibling can use the computer without having Admin powers, if you need that.
4) if your antivirus software is old stuff, get a current-generation product. For example, Norton AntiVirus 2003 doesn't support expanded-threat detection (hack tools, adware, spyware, etc). Personally, I have the hots for
Kaspersky AV Personal 5 lately, for home users anyway
5) fully configure the antivirus software, don't just install with default settings and think you're done.
general suggestions for AV config
6) if you have a router, block TCP/UDP traffic in both directions on all the ports you don't actually have a use for.
router stuffs If you don't have a router, it's not a bad way to spend $40. While it probably doesn't pertain directly to your Aurora problem, I threw it in because it can be very good at thwarting Backdoor-type stuff.