Windows won't completely load

I have an older Dell Optiplex GX260 running WinXP Pro SP3.

About 2 weeks ago I was installing software and got an error message saying an error was encountered and must close. When I tried to reboot, Wiindows only went as far as the landscape scene background, and stopped and stayed there. No icons or task bar. I tried several times, same result. I then tried booting to Safe Mode, which it did, and all my icons and taskbar appeared. I tried a few programs (Word, Excel) and they loaded and worked. I have a second HD in this computer, a clone of the first one. So I switched HDs and made the second one the primary drive. That worked fine for about a week. While I was loading software (not the same software as with the first drive) from the mfr's CD, the same thing happened. When I tried to boot, I was faced with exactly the same situation as before.

Someone suggested it might be a problem with my memory sticks, so I took them out, cleaned them, and one at a time I tried booting, but got the same problem.

I would not like to just reinstall Windows from the Dell recovery CD and then go through the agony of installing all my drivers and programs. Can someone please give me an idea of what's wrong, and - hopefully - an easy fix?

Thanks in advance to all who may respond.

 

I did that a couple of times with no success. I even did system restore to a week earlier, without success.

I don't believe it's the software I was installing that's to blame. Here's why: After I moved to the second HD I went to install the software for a recently purchased Microsoft webcam, which I had installed on the first HD. Both HDs have exactly the same WinXP, so if there was something wrong with that software, it would have shown up when I installed it on the first HD.

Regardless of the cause, is there any way I can fix the problem without having to reinstall all my programs?