Hey ... If F is your root drive ? You wouldn't be able to change it , You are going to need to reformat and delete the F root drive , recreate the Root drive and make it C , Though you can give it a try if that not the case , When I say delete I don't mean in the disk management area , I mean when you reformat your hard drive and reinstall windows , In the disk management area do not delete the F drive
Click Start, point to Run, type diskmgmt.msc, and then click OK
Have a view at this website below
http://search.support.microsoft.com/kb/307844/
HIH