Slow computer not a virus or spyware

I just got a job working on a customer built computer running xp home sp2, with a 2.something ghz possessor, and 2. gig of ram. it has a good video card. basically a computer built for speed 3 years ago. the customer said in the last couple weeks it has started running slow, . under the processes there are no major memory uses. I ran a check for viruses and spy ware with spybot and AVG and it came up clean as a whistle. the computer already has Constant protection with a program, I think it was called Norman" the program seamed to be doing its job, and not taking up much recourses.
I also ccleaner, but just the normal clean. I will use the more thurogh options and registry clean tomorrow

an error appears for a missing win32.dll(or something) file when running IE, or firefox. I can't remember the exact error name, but ill be back at the job tomorrow, I will keep a pen and pad with me from now on.

The problem of slowness is noticed in every aspect of the computer. from the slow startup, which I disabled everything so no apps start up. It seams to be more noticeable when opening the documents folder, that is huge and filled with pictures files. Or even more noticeable when trying to open IE(not updated cause customer uses firefox) and Firfox.

Also the computer is running 1.1 version of this program http://www.crystalxp.net/galerie/en.id.130.htm
it makes xp look like vista. I asked him if i could remove it, but he likes it, and I wasn't sure if it was causing harm, since he said he had it for a long time with no problems. I updated it though, but it still says version 1.1

I'm going to try a repair with an OS disk, which i never had much luck with those ever helping, but it was something i read about the .dll error message i get. will this delete any of the computers programs or documents?

Besides that I'm not sure what to do short of an OS install any ideas to help?
Thank you