This is the easiest guide to make for a trouble free install:
http://forums.g4techtv.com/messageview.cfm?catid=59&threadid=219022
Here is what we have gathered so far as the best advice for a safe install
1. Back up your data to be safe. You should have been doing this all along, but this gives a good excuse to catch up on it.
2. Do a disk cleanup, clean temp internet files, empty recycle bin and defrag. Good standard maintenance and definitly worth doing now so that things are clean when you do your Service Pack 2 install.
3. If you have an XP CD, do a system file check. This way you have all damaged or missing dlls and system files replaced before you do the Service Pack 2 install so that it is working with a solid install . Put your XP install cd in your cdrom drive and go to start/ run and type
sfc /scannow
hit enter. It will start windows file protection and compare all files to the original install or update patches(The $NTuninstall files in windows) . If it says failed , hit retry , it just means something is using it and you have to hit retry to pause the process. May take two or three clicks of retry.4. Restart computer
5. Make a restore point. Start=> Programs=> Accessories=> SystemTools=> System restore.
6. Turn off your antivirus, make sure script blocking is disabled. Enable the windows firewall (right click on your connection in network places and choose protect with ICF) and disable your current firewall. If you have spybot run and choose advanced interface (under mode) and under tools go to resident and disable resident shield and tea timer.
7. Run the install if you downloaded it or windows update if you are doing it that way. It will take some time so be patient. Do not panic if things seem to stop. Get a sandwich or something. Just check back to see if you have to click something else.
8. When it finishes and asks to restart, choose no : I will restart later and then finish. Re enable your antivirus (but leave off script blocking) and firewall.
9. Restart PC. It will take a bit longer than normal on reboot , but not extremely long. It should finish with a prompt to enable automatic updates and verify your firewall and antivirus. If Norton antivirus, it may not properly interface so you may have to choose the do not monitor my antivirus it checks for updates itself.
10. Re enable spybot resident shield. It will warn of a new IE toolbar (which warns of sites trying to run active x or install things behind your back) and a lock of the blank.htm web page file to prevent its being replaced by coolwebsearch. Re enable script blocking in your antivirus.
11. One final step I like to do is clear the prefetch file . This will make sure windows does not waste time while booting looking for files which were replaced. I prefer using X-setup to do this, but you can just locate windows/ prefetch and delete the contents.
Launch Xsetup Choose classic interface and go to system=> file system => Windows prefetch => Clear prefetch
Then click the clear prefetch at upper right.
X-Setup Pro Free system tweaking utility:
Or just go to start/ run and type
Prefetch
Hit enter.
Do an edit/ select all / delete.
I suggest reading the full article.