Hi, when you experience it running particularly slowly, press ctrl-alt-delete and you will get a list of the processes currently running. There should be a few tabs that you can select, one of which will tell you the memory used by each process and one the processor power used by each process. See if you can identify any process that is unusually heavy on the processor and memory.
If you have a suspect process - DON'T DELETE ANYTHING before you know what the process is. I have had problems with operating speed caused by unknown processes which imitate windows processes. For example you might see a process called IEXPLORE2 which is an unwanted process i.e. a virus or whatever that is pretending to be part of the operating system.
if you have this situation, installing some sort of spyware detector might be useful, as somebody else suggested. Check your modem for internet activity when you are not using firefox - if there is activity that is not being caused by you, install a firewall to identify the culprit program. It could be a legit program but it could be a nasty.
Reformatting the hard drive is actually a good idea every now and then - it's a pain but sometimes necessary.
mick