I've been having various problems using the latest Parallels build (11992) as well. Usually of the form Windows 7 not being able to start a service, Kapersky saying it's database was corrupt, or Windows explorer crashing. Most recent issue: Windows 7 detected a bad volume on startup and ran chkdisk.
I had thought it had something to do with increasing the number of CPU's from 1->2 as that was something I remember changing when the problems started. I noticed that I had also increased the memory to 2GB, so I've now dialed that down to 1GB.
So far so good, with 1 CPU and 1GB of RAM.
Running on a 2 x 2.8 Quad Core Intel Xeon Mac Pro with 10 GB of RAM.