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See if the file association fix tool helps... instructions for it's use are in the Download.com download page. You may also have to scan your system for virus and malware as the problem may have been caused by an infection. It can also be done manually (but slowly) from the Explorer / Tools menu / Folder options / File Types / select each program extension (e.g: .doc, .txt, .jpg, etc.) and browse for the program executable (.exe) to set it as the default program for the file extension.

XP File Association Fixer
http://download.cnet.com/XP-File-Association-Fixer/3000-18512_4-75738279.html

Malwarebytes Anti-Malware
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html