XPS/Dimension XPS 710 running so slow

I have the XPS/Dimension XPS 710 and when I first bought this desk top it ran very well and I loved the speed but it started to slow down and now it hardly runs I have put Reg defense on the computer and I have allways run Trend Micro. I have gone in and stoped the visuals for performance but that has not helped. The hard deive is ok and I have deleted old programs. Is any one else experiencing problems like this?  I am not sure if I have something that is running in the background that I dont know about I hate to have to take it in and have it checked if its something I can deal with. I have 4megs of ram and ntunes says they are fine. Anyone got any ideas?