|How much RAM do you have ????|
Which Process is running CPU high ???
Computer Speed and Performance May Decrease
When you open, then close programs,
programs may not free up RAM memory.
That is called "Memory Leak".
Restarting computer will clear out its RAM memory.
install more RAM memory.
Description of the Disk Cleanup Tool in Windows XP
How to Defragment Your Disk Drive Volumes in Windows XP
To have more RAM available at startup,
You can do Start, then RUN, then type
Then uncheck items in the Startup tab.
To find out what each Startup(or Process) item
does or means, and any recommendation as to if you should close the process:
Task List Programs
The Process Library
Most Virus and Spyware scans are likely to work better in Windows Safe-Mode.