How do I repair XP Home SP2?

[Hobgoblin]

After hardly ever having any problems with my laptop I find myself in the position where the damn thing won't burn CDs. No amount of System Restore attempts, swearing at the darn thing, inserting my original XP Home CD or glaring at it can solve the problem.
It seems I'm missing some files that need to be gotten from the original CD but because I have XP Service Pack 2 installed the thing won't allow me to use the Windows CD as my installation of Windows is more recent than what I have on the CD. Maddening:*
Does anyone have an idea of how to get past that so I can do a repair installation? I don't really want a complete reinstallation as I can't back up any of my work. Surely in the registry or somewhere there must be a key that tells the bl**dy thing what service pack is installed?
Any ideas?

[BOAC]

Maybe this will help? http://groups.google.co.uk/group/microsoft.public.windowsxp.general/browse_thread/thread/75fac87fe2a51776/9e4b2a9cd0077de%239e4b2a9cd0077de

When 'tis done, don't forget to 'Slipstream' your XP CD with SP2!

[Strepsils]

Find out who actually makes the CD drive in your machine, go on the web and download the latest (or any, for that matter) firmware for your drive and install it.

[Hobgoblin]

Thanks for the input guys. I will first try sfc /scannow and see if windows can decide on its own what files it needs (hopefully not a lot!) before I do anything too drastic. I'm afraid I'm not much of an expert and believe in measure twice, cut once so its going to take a couple of days to get it fixed.
Will report back (hopefully soon!)

[Hobgoblin]

Good news! I managed to do a repair installation by starting the laptop with the XP cd already loaded. The sfc /scannow did repair a bunch of corrupt .dll files but did not solve the problem, i.e. the CD drive not burning any CDs. The manufacturer of the drive (Panasonic) cannot even give me the original firmware for this drive so that idea didn't work either.
Unfortunately neither did the repair installation. Sat around scratching my head until I had the idea to see if it's just my profile or all the profiles on the thing that can't burn CDs. Wouldn't you know it...the other profiles burn. (Not perfectly as sometimes it refuses to accept the cd is burnable until I reboot.)
So now it looks like it's just settings...