For most "normal" viruses, I would check what things autoruns at startup, in start menu and in the registry key and task scheduler. Task manager can also show strange processes and point out the executable's location for you to delete. Most viruses can be removed in this way once you detect its presence, it's not hard if you have patience. Antiviruses are also good detecting those.
But if you happen to got a kernel-mode virus (rootkit), it's much more complicated, because you can't trust Windows to show you the real virus (it may tamper with all system activities). For those, the only true way is a full reformat and a clean install. Trying to repair Windows from any damage often results in an even worse system and more effort, while a reinstall gets a fresh system.