The reason might be a dying or dead motherboard battery. The time is stored in a little chip on the motherboard and that chip is powered by a small battery so it can hold the stored settings even if you turn your PC off. When that battery gets low, the first symptom is usually a slow clock. These batteries are made to last several years but depending on the age of your PC they will eventually die. The first you should do is to reboot your PC so you can enter the BIOS. When the PC first powers up, you get a black screen with some information on it. There is usually a line that reads Press F1 (or Del depending on your type of motherboard) to enter Setup. Press the appropriate key when that line is displayed to enter the BIOS.
In here you will see a lot of settings. These settings which are specific information about your PC and its hardware, are also stored in that chip on the motherboard and are lost if the battery dies. Take a few minutes and write these settings down. Then turn the PC off again and open the case. Look around on the motherboard for a small silver button size battery. On the top of that battery you should be able to see a model number. The common model for newer PCs is CR-2032 but verify this information first. You don't want to buy the wrong battery.
Once you know the model, it's time for a trip to your local electronics store. Radio Shack or any other decent electronics store will have what you need. Get the correct battery and head back home. Hopefully you left the cover off so you can dig right in, take out the old battery and install the new one. Usually they snap in and out of a little plastic casing.
When that's done, put the cover back on and turn the PC back on. Watch the screen so you don't miss your chance to enter the BIOS as described above. In the BIOS, there should be an option to Load Setup Defaults. Choose that option first. Then go ahead and re-enter all the settings you wrote down earlier, as well as the correct time and date. Reboot when you're done and make sure that you save the changes before exiting the BIOS.
Now get back into Windows and observe the clock for a few hours or days. It should behave properly now. If so, good job! You're all set. But what if it is still lagging?
Close all programs
The reason could be a program running in the background that gets loaded everytime you boot into Windows, such as a virus checker. To see what is running in the background, hit Ctrl-Alt-Del one time and you see a "Close Programs" list. Everything that you see here besides Explorer and Systray (which are basic Windows components) are additional programs that are loaded when you start Windows. Highlight them one after the other and push End Task to close that program. The dialog box will disappear each time you close a program and you have to hit Ctrl-Alt-Del again to bring it back. Just be careful not to hit Ctrl-Alt-Del again while this dialog box is still up and don't hit it twice becauset that will reboot your PC and you have to start all over. Once all the programs are closed except for Explorer and Systray, leave your PC on for a few hours or days and observe the clock. Is it still lagging? If not, then you know one of the programs you disabled is the culprit. Reboot the PC and this time close only one program, see if the clock still is behind. Do this until you have identified the culprit.
Now that you know what is causing the clock to slow, you need to decide whether you need this program to load with Windows or if you need that program at all. Depending on your decision, either prevent it from starting with Windows or uninstall it completely. However, if you need the program, contact the vendor and explain the problem. They might be aware of the problem and might have a solution for you or a patch or upgrade.
Synchronize the clock
If that is not the case or not an option, then you should consider getting a program that will automatically synchronize the clock by comparing it to an atomic clock via the Internet and making the necessary adjustment. A program I found to be good for this purpose is AnalogX's free Atomic TimeSync program that you can download from http://www.analogx.com/contents/download/network/ats.htm. You can find many similar utilities on the Internet if you want to try different ones. After downloading and installing it, your clock should now be alright as it will be corrected automatically.
Note: If you are not comfortable with any of these procedures, you should contact an expert for help as some of these steps involve opening the PC and manipulating electronic parts, which, if done improperly, can render the system unbootable.