firstly, what i normally do (i do it for spyware, but it should apply for any virus/trojan) is run antivirus programs...if you don't wanna do that (don't have one/lazy/antivirus sucks) you should goto task manager (Ctrl+Alt+Del) and check the processes list for any suspicious looking processes running on your comp. If you can't identify any, then goto regedit (Start>Run> type regedit) and click on the "current user" or "local_machine" branch (forgot what it's called, on University computer so i can't check), then choose "software>microsoft>windows>Current Version>Run" (or something similar), now when you click on the run (or run once), on the right of your screen there should be a list of registry keys...check for any suspicious ones.
Once you've found any suspicous ones, google it....type in the process/reg key name into google + process...e.g. if on your process list there was something called iexplore.exe (most possibly will be), then in google type "iexplore process" or "iexplore.exe process" (without quotes) or just without the process word. Hopefully you will find specific websites to help you remove it manually.
If you can't do any of the above, just take a screenshot of the task manager on the processes tab and i can prob help you from there