I have recently had a few issues that may be releated to DEP being enabled on my AMD series CPU. After bounding around a few forums, one mentioned that Vista may at times have problems with certain AMD systems with DEP enabled. While I do not know if that is the answer to the problem I was having, I did stumble on to a good articel about DEP on Vista at Robert Hensign's blog:
http://blogs.technet.com/robert_hensing/archive/2007/04/04/dep-on-vista-explained.aspx