Recently my MacBook started crashing very frequently. It would hardly stay up 30 minutes. I noticed that the fans would be on at full speed whenever it crashed, and other symptoms of VM thrashing, such as sluggish trackpad / keyboard response. Checking with top and Activity Monitor showed that kernel_task, PID 0, was in normal use fairly quickly consuming over 600MB of RAM on the 1GB MacBook.
Checking on a PowerBook, I saw kernel_task was using less than 100M of RAM. I didn't have a clue why kernel_task was consuming so much RAM on my MacBook, but after a couple of clean reinstalls of OS X from the original install discs (and immediate application of the 10.4.11 update) I figured maybe it's something in the 10.4.11 update. The PowerBook was running 10.4.10.
So I reinstalled again, but this time stayed with 10.4.6, which was on the MacBook install disks. Eureka! No more crashes, kernel_task stays below 100M of mem usage. But hey, the trackpad and keyboard are still freezing up every few seconds!
Then I noticed that system.log contained a lot of lines like "Dec 15 12:11:31 xanadu kernel: [Parallels] IPI stat: rescheduled 1 cpus out of 2". With the Console open I checked the trackpad for a few seconds, and sure enough, the Parallels log line would be emitted immediately after the trackpad and keyboard froze for a few seconds. To be precise, it would be emitted after every alternate freeze, but that was enough evidence for me. I moved the Parallels folders out of /Library/StartupItems, and after a reboot everything was fine. Applying the 10.4.11 update didn't cause crashes any more either.
Looks like there's a huge bug in Parallels 3.0. I just bought Parallels 3.0, and am quite miffed that it only ended up making my whole machine unusable, even when I wasn't actually running Parallels. Argh! And what the hell is with OS X 10.4.11 that Parallels can make it consume huge amounts of wired memory till it kernel panics? Considering that 10.4.6 didn't panic with Parallels installed, even though it did exhibit the trackpad / keyboard freezing. For that matter, Parallels 2.0 used to run just fine.
Anyway, if your Mac's exhibiting any of these symptoms, it could be due to Parallels 3.0. I see from this thread on Apple Discussions that quite a few people are having similar troubles on Tiger and Leopard.