how much physical memory do you have?
click start/control panel/system/general to find out if you dont know.if any less than 256mb add some more.
check that the paging file is set to "system managed size" in control panel/system/advanced/performance/settings/advanced/virtual memory/change
set as above and click set then apply and restart.
try running defrag in safe mode(tap F8 or F5 when the computer is switched on and select it from the list)or make sure there are no other programs running or showing in the tray when you run defrag.
run the disk cleanup tool also and update and run your antivirus and maybe also run a check for and remove any spyware as this could slow things down.