Disk defragmenter will not open?

For the defrag to run properly, you need atleast 15% free space. With the Windows defragger you also need to ensure that other programs dont interrupt the task. Run a disk cleanup first, next run chkdsk and then defrag. I suggest you download one of the real time automatic defraggers and let it run on automatic mode. Some defrag with low free space and also prevent a lot of fragmentation.

If you have viruses a defrag won't fix it. If at all possible, I would reformat your hard drive and start from scratch (ie reformat the hard drive and reload your operating system). If you do this you will lose everything, but you can always back everything up on CD's before you do it. I would recommend this option because I recently had a virus that slowly ate away at my computer. Virus scans, disk defrags, and spyware scans didn't catch it. Eventually my computer wouldn't boot and I was forced to buy a new hard drive. Since you still have the option to salvage you're hard drive you should do it while you can.