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.