My computer is slower than it used to be

Narrow down the problem to CPU or HDD or Memory:
right click on the task bar (very bottom of your screen)
choose Task Manager from the menu that appears
click the performance tab
in the Cpu usage box if you see 100% usage for long periods of time (say 10 seconds) you have a CPU shortage
in the physical memory box if available memory is 256 or lower you need more memory (or run fewer tasks)

CPU Problems:
-click on the processes tab
-click on the CPU heading (3rd from left) twice to sort with the highest CPU usage at the top
-if one process (other than System Idle Process) is consistently hogging more than 80% of the CPU, it may be the problem. You could click on the process and then hit the End Process button but if the process is important your computer will go wonky and you will need to reboot to straighten things out. This will also not prevent the process from starting again when you boot the computer.
- Autoruns can prevent the program you identified from running at startup

HDD Problems:
-Open explorer, right click on C:\, choose properties, General Tab, disk cleanup, check off all the boxes, OK
-Tools Tab, Check now
- Defragment Now (This could take a long time, let it run overnight)
- start run cmd [Enter], sfc /cachesize=50 [Enter]
then type sfc /purgecache [Enter]
reboot when it is convenient

Memory Problems:
- buy more memory, get whoever you bought it from to install it
- Autoruns can prevent programs that are unnessesary or memory hogs from starting when your pc boots

Internet Related Problems:
-Download, install and run M$ AntiSpyware (free) (this solves 50% of the problems i see)
-Download, install and run AVG Antivirus (also free)
-Download & install Foxy security for IE
- if you are a poweruser download and install Firefox instead of Foxy

Preventing Future Problems:
-Windows XP Sp2, built in firewall turned on