Windows XP PVM file size growing out of control?

I've had Parallels 4 installed on my new Mac Pro (10.5.8) for about a month now and today I noticed something. Originally I partitioned 300 GB of my 640 GB mac hard drive to use as the Windows XP virtual machine. The .pvm "package" file was originally a little over 300 GB at the time. In the past month I've added VERY little to the virtual hard drive but tonight I happened to notice that the .pvm file was listed as 429 GB. How did it grow to this size? Technically I really only have about 150 GB of data on the 300 GB virtual machine, but what would cause this 129 GB growth? At this rate, I'm not going to have any room left on my hard drive, and I don't know what it's being filled up with. Any suggestions/solutions? Thanks!