Two possibulities!
1) a lot of times this is a virus or caused by one...run your anti-virus and malware/spayware programs to make sure you have a clean computer...It is best to run these programs in safe mode
To get into the Windows 2000 / XP Safe mode, as the computer is booting press and hold your "F8 Key" which should bring up the "Windows Advanced Options Menu" Use your arrow keys to move to "Safe Mode" and press your Enter key.
Note: With some computers if you press and hold a key as the computer is booting you will get a stuck key message. If this occurs, instead of pressing and holding the "F8 key", tap the "F8 key" continuously until you get the startup menu.
Note: Trouble Getting into Windows 2000 or Windows XP Safe mode - If after several attempts you are unable to get into Windows 2000 or Windows XP Safe Mode as the computer is booting into Windows, use the power switch and turn off your computer. When the computer is turned on the next time Windows should notice that the computer did not successfully boot and give you the Safe Mode screen.
2)The application has become corrupted!
From Microsoft:
A supported hotfix is now available from Microsoft. For Windows XP SP2, the Generic Host Process error can potentially occur on any system at any time. Although you receive the error message shortly after system startup, the actual error occurred during the previous system shutdown. Because of the broad nature of this issue, this hotfix was made available through Windows Update and distributed to users who have Automatic Updates enabled to eliminate this problem on Windows XP SP2 systems. For Windows XP SP2, you may experience one last Generic Host Process error upon restart after you install this hotfix. This is from the previous system shutdown before the new version installation was completed.
This means that you need to download and install all the updates from M$
If you still have issues read this whole article
http://support.microsoft.com/kb/894391#SPR
