For those who run Sophos Anti-virus (4.9.19 is presently current) with a Mac OS X 10.5 system and a virtualizer like VMWare Fusion or Parallels Desktop, this may be very helpful.
It appears that a recent update to Sophos Anti-virus can seriously interfere with the operation of either Fusion or Parallels. Upon opening either application, Sophos' main on-access scanner (InterCheck) will begin to consume large amounts of processor time. Checking in Activity Monitor or via top will reveal it at the top of the list. During this flurry of activity, neither Fusion nor Parallels is responsive. After a several minute wait, both may momentarily become responsive, only to slow down again. The slowdown is most pronounced when a virtual machine is being resumed from a suspended state. Stopping the Sophos on-access scanner returns Fusion or Parallels to their normal responsiveness.
Obviously, stopping Sophos solves the problem. However, some users may be required to run anti-virus software as part of their institutional rules, so that may not be a practical option. Instead, you can instruct Sophos to exclude the location where you've installed the Windows virtual machines from its on-access scanning:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090319092122471