In order to make your computer run better do the following in this order: 

• Backup any important documents, music, pictures, contacts, etc.
• Make sure Antivirus definitions are up to date and run a full scan. If you don’t have antivirus, download and install Avast. www.avast.com (It is free but you will need to register it and install the key before the 30 day trial is up.)
• Make sure Windows critical updates are current. update.microsoft.com
• Remove any programs you don’t need. Click Start then Control Panel then Add or Remove Programs. Select the program to be removed and click Remove.
• Delete any files you don’t need from My Documents and other places you might store them.
• Cleanup your Outlook profile(s) by deleting unneeded emails from the Inbox and Sent Items then empty the Deleted Items folder.
• Run the Microsoft Disk Cleanup wizard. Click Start, and then click Run. In the Open box, type cleanmgr then click OK. Select the C: drive, then click OK. Be sure to check Temporary Internet Files, Recycle Bin and Temporary Files then click OK.
• Setup a reasonable amount of Temporary Internet File storage. Right click on the Internet Explorer icon then click on Properties. Under the General tab click on Browsing History then click Settings and set the Temporary Internet Files for 50 meg or less.
• Check the free space on your hard drive.  Click Start, and then click Run. In the Open box, type explorer then click OK. Click the + sign next to My Computer then right click on the C: drive and click on Properties.  You should now have at least 25% of your primary hard drive free (blue). If not, you will need to purchase a new hard drive. 
• Check to see how much RAM memory your computer has. Right click on My Computer then click on Properties. The memory is listed in the bottom right. It needs to be at least 512 Meg to run Windows XP. If it is below 512 Meg increase the amount of RAM. www.crucial.com
• Download and install Spyware Terminator. www.spywareterminator.com Use it to scan for and remove all spyware.
• Make sure there aren’t any programs running in the task bar you don’t need. If something is there use the built in configurations for the program to keep it from loading at startup.
• Go to the Run sections of the registry and delete anything you don’t need. Click Start, and then click Run. In the Open box, type regedit then click OK.  Click the + sign next to HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows > Current Version > Run. (Improper changing of the registry can cause fatal errors. Proceed with caution and only after backing up your files.)
• Scan the hard drive for errors. Click Start, and then click Run. In the Open box, type cmd then click OK. Type a chkdsk /f then press Enter. You will need to restart Windows. Click Start then Turn Off Computer then Restart.
• Setup the correct paging file size. Right click on My Computer then click on Properties. Click on the Advanced tab then click on Settings under the Performance section. Click on the Advanced tab then click Change. Select Custom size then set the Initial and Maximum size to 1 and a half times the amount of physical RAM. For instance if you have 512 meg of ram you should set the Initial and Maximum size to 768. Click Set after the modification has been made. You will need to restart your computer for this change to take effect.
• Defragment your hard drive using Perfect Disk. www.perfectdisk.com Do both Offline and Smartplacement defragmenting. (This program is far superior to the Windows defragmenter and can significantly improve the performance of your computer.) (You will be prompted to reboot the computer to perform offline defragmentation.)

Other tips:

• Install a second hard drive or partition on the primary drive to store documents, pictures, music etc.
• Install a second monitor. This is great for looking at your email while browsing the Internet and many other tasks. Many computers are already setup to run dual monitors and monitors are relatively cheap these days. (Once you try it you won’t go back!)
• Use a USB drive or external hard drive to backup important data. Remove the drive from the location of the computer to better protect the files.
• If using a wireless router be sure to enable WEP or WPA encryption. This will help protect your computer and make it difficult for others to access the internet through your connection. (Remember that you are responsible for what happens through your connection to the internet.)
• Turn on the on your router’s firewall and properly configure it. (See the documentation for your router.)
• Turn on Windows Firewall. Click Start then Control Panel the Windows Firewall. Select On then click OK.