rundll32.exe & explorer restarting

hi guys & gals, First time poster here, I am pulling my hair out with this problem Everytime I reboot or start up get this error message, rundll32.exe DEP has prevented code from running to keep your computer safe, please close program. and then windows explorer stops working and constantly restarts in a loop, the taskbar does not stay long enough for me to get into ctrl panel or error logs. I was surfing with firefox 3 and used down them all to download some .mpg movies & trailers to my desktop, that is when this problem started, I am going to delete the .mpg files in case this is the cause , but fear the worst looking at the forums there does not seem to be a fix for this yet, Vista is like a little excited puppy running around all over the place, and it just pees on my carpet every now and again.( vista needs house breaking :-) can anyone offer suggestions please many thanks in advance shaun

Hello,
Can you boot in to safe mode?
When the machine is booting up press F8 (repeatedly) and a boot menu screen will come up.
This screen will give you options. Select safe mode
When you are in safe mode open the control panel

• 
  open System.
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  Click Advanced System Settings. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
• 
  Under Performance, click Settings.
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  Click the Data Execution Prevention tab, and then click Turn off DEP for all programs and services except those I select.

Hi thanks for the advise, but on getting home on Friday night i decided to pull the gloves off and get down to the bottom of it. I booted up and let the virus do its stuff for about an hour constantly restarting and crashing, virus scan with avast professional came up blank/spy bot S&D came up blank/Spy ware doctor Blank/ nothing.. then I remembered I had downloaded a file to my desktop an .mpg file of some sort I cant remember so I tried to delete it and it would not delete saying access denied., Hmm i thought its not even open /or is it.? I then remembered reading somewhere about a buffer overflow vulnerability in 0.8.1f in VLC . I booted into safe mode. Uninstalled VLC and any other media player that would read .mpg files in case it was the codec. started a cmd prompt and navigated to the desktop where the blighter was installed deleted the file and wiped free space with ace utilities incase of any remenances left in the recycle bin. and rebooted. All is now well, I have been running the Pc All weekend with no problems at all… So if anyone else gets this error this is how I fixed it, hope it helps.. Regards Direct