New virus or malware?

[NotquiteanooB]

I installed Microsoft Security Essentials in May. Set it to scan each week and update b4 scanning. Noticed it has not updated or scanned since May 30. Won't update manually. Ran SuperAnti-spyware, no problems found. Ran Malwarebytes, no problem found. Noted that both had not been updated recently and tried to update; only after repeated attempts did they both finally update. Ran again no problems found. MSE won't update; won't scan; and won't uninstall !
Tried to download AVG, Panda, Avast. Error messages during the downloads, most failing almost at the 95+% complete stage. When after repeated attempts I do get a complete download, it won't install! Some error messages that "File is corrupted". Tried the OneCare package and after 2 attempts got that to load and scan ... nothing found. Checked the Processes in Task Manager and there is nothing out of the ordinary.
OS is XP-Pro SP3 and was a clean install in early April 2010. All the critical updates have been installed since then.
Any thoughts or suggestions ? (short of reloading OS again!)

Update: Just added HitmanPro 3.5.6. It downloaded, installed and ran okay. No threats, just 9 cookies; and it found no AV software installed! So either MSE didn't install properly or it did uninstall, but gave me the error message that it couldn't uninstall ?

 

[Devilpapaya]

My friend had a problem where some bad RAM was corrupting his downloads/ causing downloads not to finish. Surprisingly had no other RAM related problems (BSOD, etc) Its a longshot, and its the only case like this I've ever seen, but it could be something to check.

Beyond that; off the top of my head, rootkit? possibly HDD corruption?

 

[NotquiteanooB]

Finally fixed. Microsoft Service engineer via remote access found a corrupted file. How or why it got corrupted ?? But, all is well again. Mem tests all passed, so maybe just a passing glitch.
Thanks for the reply.