I got the same issue but I was finally able to solve the case.

I am using Vista home basic, I downloaded an "exe" file from the website http://serials-keys.com/
the link :-http://serials-keys.com/serial_All_serials_keys_text_speech_pro_ultimate_2.2.html

just after running the "exe" it asked to restart the system, once asked for system, the system was not able to booth properly. so the steps that i took were

1) start the system with "safe mode with networking", it showed me the same message as you all had received

2) Installed a trojan remover software, you can download from http://www.simplysup.com/tremover/download.html
its the best.

3) scanned my system, which showed me some virus "gxvxcserv.sys", i asked to remove from the registry.

4) Just before i restart the system, I went for folder options, and checked the option "show hidden files and folders" & deleted the hidden files that i found in the "Startup" (found in Windows--> Start Menu--> Programs or click on Start--> All Programs and you find the folder)

5) now restart the system, you might need to check for trojan virus, just scan with "Trojan remover"

this trojan remover software has saved my ass for 3 times within the span of 30 days trial.
you need to analyze what sort of problem you are facing, is it hardware or virus, go search on google if you get some text message for error, next if it is a virus than, need to know what kind of virus & how does it works, that helps to know how to fix it.

Many virus place them self in the areas with those files which are accessed by windows when it starts. like the StartUp files or you Windows registry etc.

So even if you have the finest and sharpest (best & updated Antivirus) if you don't have the skills (the way to tackle) you would always face problems.

Best of luck. If this works, kindly spread this to all other websites and do buy the Trojen remover software.