With certain other programs-- in particular it will not work well with Prevx (learned this the hard way, posted on the Prevx support Forum at Castle Cops and discovered it was a known issue).
My computer is a bit more moedern than yours (!.8 gig processor running XP) and it could not handle SD and Prevx together. But, even super-charged pcs I discovered had the same problem. They conflict. Spyware Doctor is no longer loaded in resident memory for me (I use Spyware Blaster and Spyware Guard in addition to Prevx) and I just use SD to scan out baddies (course, I don't get many of those, using all that prvenention and FireFox as my primary browser!).
If you're looking for a spyware program, you may have seen that Spyware Doctor has received PC Mag's editors choice. I installed it on my computer and had all kinds of performance problems such as 100% CPU usage by Spyware Doctor, Internet Explorer failing to close when I clicked to close it, and some sites (Yahoo mail for example) running slow or not at all. Granted my computer is 5 years old, but it runs Windows 2000 and all my other programs without a problem. So for now I've gone back to Microsoft Anti-Spyware with which I've had no performance problems.