You'll have to try to work out exactly what it is that's slowing your computer down.
To try to track down what's slowing your computer, as soon as you start it open the Task Manager (via right-clicking on the Task Bar at the foot of your screen). If the 'Processes' tab isn't already selected, click on it. Then click on 'Show processes from all users'. Maximise the window (by clicking on the square in the top-right corner) and then minimise it down to the Task Bar (by clicking on the line in the top-right corner).
As soon as things slow up, bring up the Task Manager window again and look for any of these signs of trouble:
1. A high number in the 'CPU' column. (The only big number should be against 'System Idle Process'. Everything else should show '00' or else be in low single figures).
2. A high number in the 'Working Set (Memory)' column. (Most figures should be under 10,000K. A few larger ones should still be under 100,000K. Only your web browser, or other major programs, should be over 100,000K but still below 500,000K).
3. A number in the 'Working Set (Memory)' column which, even if it's not very big, keeps changing every second or two. (That's particularly important if you've already seen a high figure, in either column, against an entry for 'svchost.exe', as it reveals what's causing 'svchost.exe' to become so large).
If you find any of those symptoms, post here to tell us about them, so that we can suggest a solution.