FYI:
My E: and L: partitions had been disappearing on my hard drive about once a day for the last two weeks.
Norton Internet Security's full scans found nothing.
Two days ago, my computer started running very slowly. Task Manager showed a very large svchost.exe process labelled "winrscmde".
A google search led me to TDSSKiller.exe at
http://support.kaspersky.com/faq/?qid=208283363
which found a Rootkit.Boot.Pihar.c on my computer.
After TDSSKiller.exe got rid of the rootkit, the problem disappeared.
Pete