Repeated System Crashes

In the last couple of weeks I have been absolutely swamped with computer problems. I have downloaded and run numerous virus and spyware programs and have found a number of items, but cleaning them appears to have no effect. I constantly get a Windows popup message that says "program -XYZ- has encountered an error and must close" that also asks if I want to send a copy of the error report to Microsoft. When I bring up the error report, it tells me that the module which has failed is unknown. I have tried several times to download the Windows service packs and every time I try I get an error popup that says the system will shut down in 60 seconds and says that it is C:\WINDOWS\system32\services.exe that is causing the shutdown. My homepage has also been hijacked by a-search.biz and I have tried to follow all the suggestions I have seen to remedy this, but every time I do anything it only lasts until I reboot and then Spy Sweeper informs me that the page has been changed yet again. Any help or suggestions to get my butt out of this jam would be greatly appreciated.

Click here to download HijackThis by Merijn Bellekom. Doubleclick the file, click Unzip and it will save the application to C:\HijackThis. Run it from there to scan your computer.

When the scan is finished, the "Scan" button will change into a "Save Log" button. Save the log, Ctrl-A to Select All and post it here for examination. Don't fix anything yet as most of what it lists will be harmless.