I've just spent five hours trying to figure out why my XP desktop is so slow as to be unusable. Windows Task Manager took five minutes to appear and pointed to 99% use by svchost.exe (SYSTEM).
Must be a virus, I thought but I couldn't run Malwarebytes as the blasted PC was virtually dead.
Fortunately I also have a Win7 laptop and eventually I found the cause of the problem - MICROSOFT and the Windows Update.
Details are below - any one else had this??