I have 10 pcs of the hundreds I care for that slow down to the point of being unusable - sometimes it clears itself after one to several minutes, sometimes you must reboot. Task Manager shows it to be the SVCHOST.EXE process (which you are sometimes allowed to kill and sometimes not.) Looking into it with Process Explorer shows it to be the following thread: ntdll.dll!RtlAllocateHeap+0x18c. It will climb up beyond 98% of CPU time, and the Context Switches climb like mad as it runs. You can both kill or suspend this thread with Process Explorer.
I have one machine in front of me, and the svchost.exe process actually contains 3 of these identically-named threads. Only one is the culprit, however - while I suspend it all is well, and I resume and it consumes the CPU - none of the other threads use any CPU time whether the bad one runs or is suspended. (I have not looked to see whether the other machines have multiples of this process, but in all cases, there is only one that goes crazy, and it is always called ntdll.dll!RtlAllocateHeap+0x18c.)
My configurations vary somewhat, but all have SP2 on Windows XP and the latest patches delivered via Windows Update, plus versions of MS Office and Symantec or Norton AV. For anti spyware, all the XP machines run Windows Defender. I have had one machine show this even with the OS freshly loaded from the Dell Recovery Disk, before I loaded Office and the standard utilities. I have too many people suffering too much pain for this, and am really hoping for an answer. I have to believe other folks are seeing this as well, although I've searched EE multiple times over the last couple of months I've been seeing this, and have only seen a post or two, with no definitive solution.