Parallels v4 for Win XP Pro - painfully slow

I am running Parallels v4 (Build 4.0.3848) on a MacBook Pro, Snow Leopard OS. I use coherence mode and tend to use it for either AutoCAD lite or Photoshop.

My problem - My MacBook Pro slows down to a crawl as soon as I start Parallels. Booting a program up can take up to 5 mins. I'm not joking, it's true. I have no idea what I have done wrong but I am hoping someone can advise me on where to look (I've tried and had no success, even RTFM).

As I have a dual core processor and 4gb of ram I must be able to improve this.

Hope you can help.

Mark