Hey AjIsCoool,
try motherboard monitor 5, it will monitor your fan speed, cpu temp, mobo temp, etc., you might also check your hsf to see if they haven't come loose, chipsets as well as cpu, clean out your case and fans if they are dusty, use a software hlt program like vcool, also your question if you should upgrade cpu, you can only upgrade based on the type of cpu you have, ex: slot to slot, socket to socket unless you buy a converter like a slocket to go from slot to socket, the p4 will not fit unless you buy a new mobo, which may not fit in your current case. If you decied to go this route (which probably is preferable to what you have) you should go the whole route, etc, new case, new hsf, new ram, etc. most of the other parts you can reuse such as drives, sound, video, modem, etc.
On why you computer is so slow, have you checked bios settings to see that all is correct? such as DMA or Ultra DMA enabled?, video set to PCI or AGP? etc., also check to see your device manager if there are any conflicts, switch over to controllers and see if you have any running in pio mode, switch them over to dma mode, as someone else said go to msconfig and shut down all programs that you don't need, RealPlayer, AOL, Screensavers, etc. And turn off that norton av, use it only for maintenance and/or file checking, running it in the background will slow you down significantly, you want a good av that doesn't slow you down try AVG or TrendMicro, as the other posters advised get adaware and clean out your system from spyware/adware/etc. and free up some bandwidth that way, get a good firewall, zonealarm, tiny are a couple of good ones that you block programs from getting online and sucking up bandwidth as well as installing/monitoring/tracking you, your hard drive, and your websites.
I would also clean out your hard drive, much as another poster suggested, you can do it manually (not suggested if you don't know what you're doing) use a program like window washer 4.5 or so, cleans out excessive webpages, cookies, junk files, etc, and erases them securely, or go to langalist.com and search for a freebie called clean all.bat, it does as much if not more and for free, no secure erase, however if you've never cleaned your hard drive then this could free up quite a bit of space (maybe gigabytes, definitely megabytes).
Also uninstall uneeded programs, clean up your registry with regclean, regcleaner, etc, lots of freebies for this, compact your registry with scanreg\opt switch, this will enable quicker loading of windows and faster performance because registry doesn't have to load junk/nonexistant programs, dlls, etc and will be smaller.
After all this do you system maintenance and you should see a significantly faster performance, will this solve your specific problem? Maybe, maybe not, but it will make it easier to dx and will give you a performance boost.
Keep in mind these are just the basics, theres more tweaking you can do, also problems that you need to resolve. You might want to put in a more descriptive question to get a more specific fix.
Good Luck!