From: http://theinquirer.net/?article=31281 

There are 2 parts to this spyware popup: WGAtray.exe and wgalogon.dll. Killing the WGAtray.exe process causes it to reappear in 1 second. With it present, WGAlogon cannot be deleted. And you can't delete it while it's running. Seem impossible? Nah.

First, you need to have an Explorer window open and pointing to the C:\windows\system32 folder, where the spyware resides (interestingly, doing a hard drive search for "wgatray" turns up nothing- clever spyware, this is!). And you have to have Task Manager open, right beside the Explorer window. This is tricky, and must be done fast- you kill the process in Task Manager, and before the spyware can reopen itself, you must delete WGAtray.exe in the Explorer window. You only have a split second, but it is possible. Once the spyware .exe is gone, you must reboot your computer.

Yes, now you will see the spyware's nag screen. No problem, it's the last time. When you DO finally get to the desktop, do a search "for WGAlogon.dll" You will find 2 copies, one in system32 and one in the dllcache folder. Without the WGAtray spyware to protect them (did you notice it was gone? YAY!), you can rename and then delete both these spyware .dll's. Your system may hang when you reboot it the first time, but when it is brought back up, THE SPYWARE IS ALL GONE. 
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Following that, I went to the regular command line and deleted the files & directory found at - C:\WINDOWS\SoftwareDistribution\Download\6c4788c9549d437e76e1773a7639582a

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From the same article, here's another method that I've not tried:

The annoying pop-up (WgaTray.exe) can easily be removed in following steps:

1. run > regedit, search and delete all references to WgaTray.exe

2. start in safemode and remove the infection by deleting the file WgaTray.exe in c:\Windows\System32.

3. restart your computer and enjoy your fredom, dear rebel

> 4. start using linux or mac 

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Another article about deleting this spyware. It seems a bit complex as compared to method #1 above. http://labnol.blogspot.com/2006/04/workarounds-to-disable-non-genuine.html