Your computer slowed down just after installing service pack ?
Yeah ! I too faced the similar problem .
I installed service pack on my windows 7 and as soon as I restarted the computer , Processor usage reached 50% with no running applications.
Finally found that the cause for this was a process called mscorsvw.exe process . This process is responsible for compiling .net assemblies .
When we installed the service pack which includes the .net updates or pacthes , it will re compile the assemblies in the background.
Once this process complete ,the processor usage will return to normal.
If we run the command ” C:\WINDOWS\Microsoft.NET\Framework\(version of framework)\ngen.exe executequeueditems ” It will forcibly process the queued items.
On my computer the active ngen.exe was in v2.0.50727 so , I just right clicked on accessories –> commandprompt and choose run as Administrator , then typed the command