Since installing XP Service Pack 2, the few updates that have occurred subsequently have all failed to install after downloading. Downloading seems to reach 95% completion normally and then slows right down. 'failed' then appears against the update name.

Make sure your firewall settings are not to high,otherwise you may experience problems.If you use dial-up to connect to the net,make sure you have a good download manager installed.Once sp2 is installed,even if you get cut off and have to reconnect to the net,your downloads should pick up from where you left off.
I know you probably have,but check your anti-virus software,use spybot 1.3,Adaware,clear old useless progs.from off your PC,defragment your hard drive at least once a month,turn off picture display on your desktop,to free up more processing power.Yes a tiny jpg image may look nice,but you'l be surprised what happens to your PC on start-up.
If you use broadband,check with your isp that engineers are'nt in your area.Check all your modem connections yourself.
Like a lot of Windows apps, it's a case of hit and miss.Some people have no trouble,whilst others hit a brick wall.The extreme measure would be to delete your copy of sp2,and get another.Annoying I know,but just a silly thought.
I hope this helps a llittle.
Good luck.

Those are good suggestions to follow above, also try some of these programs and ideas.

For Spyware protection and general help cleaning up the computer and registry I recommend, the following files, as I have found 1 or even 2 is not always enough as 1 may find something the others do not.

Spyware
Microsoft New SPyware program-Formerly Giant spyware, they bought it out.
Spysweeper
Spybot search and destroy
Adware SE
Aluria's Spyware Eliminator
Spyhunter

I would suggest at the very least 3 or more of the above programs.

Registry and Fix and Repair programs
Fix it Utilities 5 this one has an excellent Scandisk, better than the one with your OS
Advanced System Optimizer
Ace Utilities
System Mechanic 5

Any 2 or more of these.

And download the Google toolbar with built in pop up stopper for your browser is good.

And a good solid Anti Virus Program or 2, updated weekly.

A good defragging Program or 2
Diskeeper Lite
Perfect Disk.

Also run from Start/run type in chkdsk then let that run then type in chkdsk /f to help fix WinXP errors.

Also use the below website, that tests and reviews some for the best programs out for many different things, they give you side by side comparisons and rate each one in many categories.
http://www.toptenreviews.com/