i hope the following advice will help:
the previous posts about getting a firewall, antivirus etc needs to be followed urgently.
the hackers have probably left a backdoor in your system (actually, if they know what they are doing then there will be many backdoors).
installing a product like zonealarm will help you a lot as it will tell you when an application is trying to get through to the internet. you can then allow or deny the program. be very careful though, because some backdoor programs can be renamed, so if internet explorer is trying to get to the internet ask yourself "did i launch internet explorer? is there any reason it should be trying to connect to the internet?"
(NT/2000)have you checked who's in the admin/power users etc group? is it everyone that you know to be correct? is there any extra accounts?
have you checked the startup folder for strange programs?
check the following registry keys for strange programs:
HKLM\software\microsoft\windows\currentversion\run*
get the latest virus dat file for your antivirus product as they also pick up nasty trojans like Back orifice.
the bad news is that it's possible you can't clean your system completely. If you decide that you are going to reinstall then please please please
DON'T use the same username /password combinations.
DON'T restore from a recent backup (you'll just end up restoring the trojans as well, and you don't really know how long the cracker has been using your system)
make sure you are up to date on all the service packs and patches. Microsoft has recently updated thier security patch section. It is now much easier to see what patches you require for a combination of OS and service packs URL...
http://www.microsoft.com/technet/security/current.asp
hope this helps.