My computer will not let me defragment my c drive?

If this is XP, it usually is the "check disk for errors" utility that advises you it is scheduled. The reason is many system files are locked. It runs prior to booting to get around this. Defrag will just skip these files. There are a couple of possible reasons it isn't working. First, if your disk is nearly full it may not have enough room to work in. Second, do a disk cleanup first and get rid of all the temporary Internet files. Another more complicated reason is the pagefile could be corrupted. There are a couple ways to fix this. The simplest is to set Virtual Memory to zero and reboot, then reset it back to the original settings. You need to be logged in as administrator to do this. If this doesn't fix it, from start, run, type "sfc /scannow" (no quotes) and click OK. This is the system file check utility and it either repairs or replaces missing or corrupted system files. You will need the XP install CD for it to get new copies of missing files. Hope this helps, I don't understand what you mean by "please run the f drive". Add some more detail if this doesn't work.