You'll find XP's defragger in the same place as the Error-checking tool-- My Computer / right-click drive letter / choose Properties / Tools / Defragmentation / Defragment Now...  Three more ways to start Defrag are:
    - Start Menu / Run... / type dfrg.msc / click OK or press ENTER
    - Start Menu / Run... / type defrag -f c: / click OK or press ENTER
          c: represents the drive to be defragged, -f forces a defrag, even when disk space is low
    - Start Menu / All Programs / Accessories / System Tools / Disk Defragmenter

This has become my favorite way to defragment hard disks.  It's a script that does all drives with one command.  Check it out here.  This tool is courtesy of Doug Knox who owns and operates one of the very best XP resources on the net.

The same old rules apply-- close down as many programs and remove as much file junkage as you can before using Defrag... Disable Virtual Memory, the screen saver, Power Management, your virus scanner, modem dialer (if you're on dialup), the firewall, web browser, word processor, emailer etc.  If you have a cable connection, don't forget to plug out the modem before shutting off your firewall.  An off-duty firewall means a virtual open door to your computer-- unless you disconnect the modem as well.

More on Windows XP's Disk Defragmenter:
    - Disk Defragmenter Utility (The Elder Geek)
    - Why defrag? (Geek Girl)
    - Defrag (Microsoft)
    - Disk Defragmenter & CheckDisk in Windows XP (Kelly's Korner)