Andrewr, I've reported this problem to the Parallels support folks several times. You are not alone. And, it is not limited to Vista - I am running XP Pro and it suffers the same. I don't seem to recall this happening prior to build 3188.
I have the freeze problem also. MBP with 2Gb (1Gb to Parallels) running XP SP2. Locks the entire machine including all OSX apps. Spinning beachball etc. Currently running 3186 because of the problems I've seen reported with 3188.
Running 16Mb of video memory
Generally that Mac OS spinner and hangs indicate that you are using too much memory for both Mac OS and VM, causing the system to use the swap file constantly.
It's better to keep VM allocated memory at the recommended level (it wouldn't be called recommended without serious reasons ). You can also try switching VM cache optimization policy to Mac OS (go to VM Configuration Editor - Options - Advanced), it will degrade your VM performance to some level, but it may also improve the Mac OS performance when VM is running.