[Blind-sysadmins] Malware taking over PC's

I'm looking for advice and suggestions on software tracking down malware which appears to be using a PC to perform massive downloads. To describe the issue, Virgin are claiming that a system has downloaded over 700 GIGA bytes on a 50 GB cable connection. The machine is never left on overnight or for extended periods of non-use. Sadly, Virgin a not being very helpful. Any ideas? George.