DAZ,
>>>I usually do it once a month. But a word of warning, if you have not done it in two months or more, dont bother, it will actually slow your system down. <<<
What that will do is slow down how long your defrag program will take to reorganize your hard drive. Defragging certainly will not slow down running your programs. Just the opposite. Your programs will start and run faster. you will see much less hard drive activity when running and accessing programs and files.
I suppose also those who do not defrag also do not run any programs like Norton's DiskDoctor to check for hard drive errors. One of our club members got a call from a computer user that he could no longer access programs. When my friend checked out the hard drive with DiskDoctor it found litterally hundreds of hard drive and file errors. What a mess!
After running DiskDoctor several times he finally got all those errors cleaned up. Problem was so many files that were corrupted belonged to programs so to get those programs to run again they had to reinstall them.
Keeping your hard drive healthy is a must do thing not a second thought type of thing to do later.
JL