Try starting your computer in Safe Mode. (Press F8 during the boot process to access it. Use the 'Safe Mode with networking' option). Everything will look rather odd (because your graphics driver won't have loaded) but you should still be able to do some basic tasks. Give them a try. If everything is then running at a 'proper' speed, you'll know that something that usually runs at Startup is the cause of your problems. (If everything is still slow you've probably got a hardware problem, such as dead memory, which might mean that it's time to buy a new computer).
Assuming that things are working at a sensible speed, restart your computer to get it back into its normal way of working. Enter the Task Manager (via right-clicking on the Task Bar at the foot of your screen). Click on the 'Processes' tab and look for anything suspicious, viz:
1. Anything (other than 'System Idle Process') that has a high figure in the 'CPU' column ;
2. Anything that has a particularly high figure in the 'Mem Usage'' column; or
3. Anything in the 'Mem Usage' column (even if it's a low figure) that's constantly changing, second by second.
If you find anything suspicious, please tell us what it is.
Assumining that you've not found anything there, go to Start > Run, type 'msconfig' into the box and hit 'Enter'. Click the 'Startup' tab to see what's running automatically every time you start your computer. Deselect anything that doesn't look as if it's absolutely essential, click 'Apply' and restart your computer. (You'll see a warning telling you that you're using 'Selective startup'. Accept the 'Do not show this message again' option). Hopefully your computer will then run normally. If so, go back to msconfig>startup and reselect one of the processes. Then reboot and see how your computer is running. Repeat until you find the process that's causing the problem and then tell us what it is.