XP's file compression is much better than in previous versions of windows. I do remember he hell that was windows 95-98 compression and yes it caused a lot of people a lot of grief. As for it corrupting, the simple answer there is back it up. Honestly a little lag inside poser is going to happen whether the files are compressed or not. Hell poser itself is just flat out slow, well P5 is anyway. If you are hurting for harddrive space then compression is the way to go, if HD space is no problem then I would leave it uncompressed.