What would cause my cpu usage to hit and stay at 100% stopping me from opening any other programmes?. Has happened a few times and I've ended up having to reboot

All I had running was the internet, Kazaa, real player and I also had explorer open,

Has something got in and is clogging up my system, can i clear it out somehow.?

Ta

What operating system are you running ?
If it's windows 2000, right click on the toolbar at the bottom of your screen and choose task manager from the popup menu. Click on the Processes tab and then on the CPU column so that the application that uses the most cpu appears at the top of the list. This will tell you what it taking up your CPU time.
Windows 98, if you CTRL+ALT+DEL, one application may appear as "not responding", that would suggest it is the culprit.
I don't run ME or XP, so don't know about those.
HTH

AS said above run task manager and see what process is running at 99%.

Have seen this before, worse thing is you downloaded a trojan or virus that causes the cpu to overload, first do a virus scan then use something like spybot or any anti-spy program, check your task manager to see what programs are running, disable any you can think of that should be running.

had this problem so ran a program called spyhunter un told me shit were in me registry that shouldnt hav bin so deleted them un no trouble since

As above, run at least 2 virus scans, and ensure that both your firewalls are running

Also on CTRL+ALT+DEL do you have loads of programs? If so do this:

Start - Run - type 'msconfig' - Ok - Startup tab across top - Uncheck them all - Now ensure 'systray' and 'explorer' are ticked plus any anti-virus program - Ok - Restart

Should free up some memory and so make it less of a problem for your CPU