Here are the most common problems and their cure.

1. Computer Freezes Periodically.

The Fix:
A: Hit the Ctrl Alt Del keys to bring up the Task Manager. Select the Process Tab, choose Exlporer.exe and click End Process. Ignore the message.
B: Stay in the Task Manager but this time Select File, New Task (Run) and type explorer and click OK.
What Happened: Windows Explorer is no where near perfect and can cause intermediate freezing for no apparent reason. The fix reloads explorer and by doing so rids itself of the problem.
A More Permanent Fix: If this behaviour continues consider using a third Party Explorer Replacement such as Explorer2 lite, FileWorks XP or Explorer XP. 

2. Computer Continues To Freeze.

The Fix: The usual suspects are the Temp, Internet Cache and a failing Hard Drive
A: By Open and use Window's own cleanup tool you can get rid of all the suspects. The Tool is called Disk CleanUp and is one of many Windows System Tools. Access these via the Start Menu under All Programs>Accessories. After the cleanup is finished run the Check Disk Utility. Have your Windows Installation CD Handy.
B: Open My Computer, Right Click the appropriate Drive, choose properties then choose the Tools tab.
C: Click Check Now in the Error- Checking area.
D: Check both Automatically Fix File System Errors and Scan For and Attempt Recover Of Bad Sectors options then Click OK.
E: Run both your anti virus and anti spy ware scanners
F: Fix any ActiveX problems by opening the Control Panel; choose Internet Options then under the General Tab/Settings/View Objects right click to delete any items marked "Damaged" in the status column.
G: Last but not least run the good old System File Checker by clicking Start then Run and type in cmd and hit the enter key. In the new Command Line box at the prompt type "sfc /scannow" (without the quotes). Make coffee as this scan may take a while. After all that and you are still having problems then you have gone as far as a typical user can go. Now is the time to ask the questions or seek further help. When asking a question informs a forum or whoever that you have tried everything listed here so whoever provides advice is not covering old ground.

3. PC boots up to a Black Screen with a message that my Hard Drive does not exist.

99% of the time this is caused by a dying or dead CMOS battery. CMOS is where all the basic BIOS setting are stored and when you power off the system the battery provides enough power to keep that data alive. Lose the data and the system has no idea what to do when it starts.

The Fix.
A: Before this happens access your CMOS settings by holding down either the Del or F2 key when you boot up. There will be a line on the bottom of the screen that says Press Del or F2 to access setup.
B: Once in CMOS go to each page and write down the settings or use a digital camera to take a pic of each page.
C: Buy a new battery. It is wise to replace this battery every two years whatever. Although there are utilities around that can backup CMOS settings having them written down of photos of each page is far more secure, as long as you do not put the information in a "Safe Place" To help you along with these three basic steps to get a system back to working order you can be assisted with some extra useful tools.

For analysing deep problems FileMon, RegMon and Process Explorer are three excellent free tools available at www.sysinternals.com
To repair the CMOS a step by step tutorial can be found at www.liverepair.com/encyclopedia/articles/cmosreplace.asp .
Now if nothing works and you start looking at the hammer as an alternative fix tool it is time to strip the system back to its bare essentials, reformat and start afresh.