Format/Reinstall is always best.. but that can take 90min to 10hrs depending on how many programs and how many patches you need to reload for your software.

If the computer isn't highly used for online banking, credit card shopping online, etc.... and is just used for gaming/surfing.. then cleaning it up may be an easier option.

If a friend does alot of banking/secure stuff on their computer.. I always recommend a reinstall, because these can leave backdoors and other things as well.... once your infected, you never truely know if the infection allowed someone to put something "custom" on your computer. Something that anti-products won't pick up, and something that could be a backdoor for all your information from now on, despite the system behaving normally.

Format/Reinstall will get rid of all that. Only thing that could remain if you wipe everything, is that your IP address could still be on a spyware/hacking list somewhere, and as soon as you get the internet connection working again... you'll be hit with a bombardment. A router/NAT solves this problem however.

Patch the system and keep it fully up to date.
Configure Internet Explorer more securely.. by default 6 was very unsecure.. 7 says its better.... but I dont know.
Dump Internet Explorer and grab Firefox.
Install AdBlock for firefox
Install Noscript for Firefox.