Windows 7: Server Execution Failed

I got windows 7 home basic 64 bit with my dell pc this month.
It was working fine till yesterday.
Now whenever I try to open any media file(any codec) I get the error "location of media
server execution failed".
There are even three other problems like I try to transfer media from my pendrive to my pc I get the access denied error.I have already tried to switch of UAC no effect.While
starting some programs I get "Windows security warning " ActiveX control and something else.
Third Error-My firewall is always off and I am not able to remove McAfee trial version.
Please help will be grateful.

Sounds to me like you may have inadvertently installed malware, probably in a media file, or perhaps by browsing with no firewall on. A common variant of a FAKE antivirus program will behave just as you have indicated.

Install and maintain a quality AV solution, never use media from unknown sources, and turn your firewall on.

thanks but my firewall is supported by Mcafee and McAfee does not work properly.Got the trial version along with the pc.Any antivirus you recommend.

That question is well debated in the proper section of this forum:
System Security - Windows 7 Forums

I use and recommend Microsoft Security Essentials and secure browsing habits:

http://www.microsoft.com/Security_Essentials/

1-800-PC SAFETY (Microsoft's free support line for security related issues)

Microsoft Security

Also note that Windows 7 free support is ongoing seeing as it was just officially released.