Slow Windows Xp Home Shutdown

Could be a number of reasons.

Is the Firewall on?

Have you an antivirus scanner running?

A large number of applications running when you are shutting down?

Have you scanned for malware?

Make sure all your drivers are updated and compatible with XP SP2.

Make sure you run Anti-Spyware such as adaware

Also have a look at this link

http://aumha.org/win5/a/shtdwnxp.htm

this is pretty common problem and can have many causes. since it cropped up for you after sp2, i'd be looking at old drivers (nic?), suspend mode, old bios, older programs especially pc doctor or utility types or cd burner especially drag and drop cd burner programs need to be updated after sp2
you might check the event logs to get more clues. any bangs in device manager? does it shutdown quick any way? like in Safe Mode or with the network cable unplugged....

Resources to help troubleshoot shutdown problems in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;308029

i have not heard of this as related to sp2 except maybe you need some driver updates now that are compatible with sp2. there is a section on that in the above doc.