Your computer is in a network?
Make some cd-s with drivers, antispyware, microsoft updates (eventually), some of the things Raptor told (I recommend you Avast! as antivirus) and repeat the windows installation without connection to network (unplug the network cable from your computer!).
So... install Windows with drivers and applications, install antivirus, install firewall and after that you may try to plug in your network cable.
I don't know what to say else. It could be a faulty Windows installation, it could be a virus. If you don't have important data (or much data) on your harddisk, better recreate the partitions (delete partition, recreate partition).