The FAH client has a virus in it?!

From FaHWiki

Some virus scanners will find/suspect viruses inside F@H data files. None of these reports have ever been proven to be valid and are just a false positive reports.

AVAST is a known offender for false positives. Always get a second opinion (try another AV scanner).
NOD32 has also been known to block the F@H client from uploading/downloading though not always prompting about a virus.
Norton Anti Virus has also been known to delete the whole client and uninstall it when it tries to run if you have not added an exception to it.

Reference Links

• Folding-community: Virus detected in WU
• Folding-community: virus found