I pretty much agree with the previous post. My update procedure is as follows:
1. Removal of any 'unnecessary' software. Especially internet junk. (games, Internet Explorer add on toolbars, Search Assistants, etc....)
2. Complete "Safe Mode" scan with updated anti-virus software and a spyware remover.
3. Boot normally. Remove all Temp files, temporary internet files, and empty Recycle Bin.(Disk Cleanup utility is good for this)
Run the Registry cleanup utility. DON'T delete any entries that you are unsure of or don't know what they are. Take your time going through them.
4. Defrag the hard drive.
5. Unplug all peripherals except for the basics.(mouse, keyboard, monitor)
6. Run the Service Pack install
7. After installation; plug in peripherals one at a time, making sure each one works before plugging in the next.