1. Step 1
The first place to look is check hard drive free space. Ever see how after a minute of absorbing information, you begin to stop understanding anything because brain is full? If PC hard drive gets full then similar thing happens, it begins to process information slower. Here's what you do:

Double click on My Computer icon, or single click it if it is in Start Menu.
Right tick on Local Disk C: and select Properties. In this you can see how much free space is there on hard drive. You can select Disk Cleanup to vacant unutilized files, and then go to step 2, defragging hard drive.

2. Step 2

Hard disk should be defragmented. This means organizing hard drive. In Properties window, tick on Tools tab and then on button that shows Defragment now.... Then tick on the Analyze button. There will be message that will show whether or not the drive should be defragmented. Follow that advice.

3. Step 3

Check for viruses or spyware. Many times PC will slow down as it has become overloaded with viruses, spyware, malware, and other programs collected like barnacles as surfing the Internet. Many anti-virus programs are there, with programs particularly intended for spyware. Ensure you have an anti-virus program installed and that it is current. You can also download a program like Spybot to search out spyware and remove it from system.

4. Step 4

Enlarge the page file size. It is an area of hard drive that PC utilizes when RAM is all filled up. It's not as quick as RAM, but it does help with speed of processing.
Right tick on My Computer and go to Properties, then tick on Advanced tab. In Performance sector, tick on Settings button. In next window tick on advanced tab then below Virtual Memory, tick on Change button. Then select Custom Size radio button, and then enter in numbers like screenshot.
When this is completed, tick on Set, and then OK, OK, OK to close all of the windows. Restart if you are prompted.

5. Step 5

Add remove programs. This needs some "letting go" skills, when it comes to programs that you want to use, but have not touched for a moment. Just like house can turn into cluttered with unused stuff, so does PC.
Click on Start Menu, and select Settings > Control Panel, or just Control Panel if it is shown. Then double tick on Add/Remove Programs. This shows a list of all programs that are installed on PC. Search through the list and uninstall any programs you do not utilize any longer, or do not want. You can also look for any files, documents, pictures, etc. that you do not want any longer. These files can be removed, or saved to a disk that will obtain lost over then next two years.
After removing all programs, repeat steps 1 and 2.