The minimum... well that would be (for WinXP)
-Explorer.exe
-Taskmgr.exe
-SpoolSV
-SvcHost (four instances)
-Lsass.exe
-Services.exe
-WinLogon.exe
-CSrss.exe
-Smss.exe
-System
-System Idle Process
At least that's what I'm fairly sure is the "safe" amount to run with. Kill everything else.