Help Required - Virus Question

My computer is at least 4 years old and I am running Windows XP. I have a 
virus which appears to somehow convert .exe files into .Ink files.
Naturally 
the files, and therefore the programs, will no longer run. What is the
name 
of this virus and is there a specific tool or patch to remove it?

Also if I download the trial version of the NOD32 anti virus program, do a

scan and it finds some nasties, do I have to buy the full version before
it 
will attempt to deal with them? And do I download the 32 or 64bit version?

Which is best for me?  Any info appreciated. Thanks.