How do I reinstall Windows XP without erasing my installed programs?

My Dell Dimension 8400 running Windows XP is not allowing some programs (previously active) to open. This includes some of the Windows native programming. I will get messages like ".... is not a valid Windows image" and "ordinal ... could not be located in the dynamic link library ....". The Microsoft support site was not much help. I believe I need to reinstall Windows XP, as even the "restore" function will not open. Is it possible to perform a reinstall without erasing all the programs I have already installed on my hard drive? I do have all the proper discs for reinstallations, but didn't want to have to go through the exercise if not needed. Please don't scold me about not having a full backup, as I have already done that extensively myself (external backup drive is on the way from Amazon)! Any help will be very much appreciated.