(Vista help is a great place to start for self help on troubleshooting PC problems… Below is a snippet from Vista Help on how to scan your hard drive for errors)
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Check your hard disk for errors

You can help solve some computer problems and improve the performance of your computer by making sure that your hard disk has no errors.

Double Click to open Computer.

Right-click the hard disk drive that you want to check, and then click Properties.

Click the Tools tab, and then, under Error-checking, click Check Now. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

To automatically repair problems with files and folders that the scan detects, select Automatically fix file system errors. Otherwise, the disk check will simply report problems but not fix them.

To perform a thorough disk check, select Scan for and attempt recovery of bad sectors. This scan attempts to find and repair physical errors on the hard disk itself, and it can take much longer to complete.

To check for both file errors and physical errors, select both Automatically fix file system errors and Scan for and attempt recovery of bad sectors.

Click Start.

Depending upon the size of your hard disk, this may take several minutes. For best results, don’t user your computer for any other tasks while it’s checking for errors.

Note
If you select Automatically fix file system errors for a disk that is in use (for example, the partition that contains Windows), you’ll be prompted you to reschedule the disk check for the next time you restart your computer.

Sometimes you  need to run a program that needs all available resources of your PC. For example running a benchmarking program on your PC, or after you do a new Windows install you want to force all of the background processes running in the background to run and complete their task.

To force background tasks to run that are waiting for the Windows PC to become completely idle you can run at a command line the following command.

You may need to be a local administrator in order to run the following command.

Click on Start and select run. Type in cmd (which stands for command) and click OK.

You will see a console Window, which is by default black.

At the prompt type in:
rundll32.exe advapi32.dll,ProcessIdleTasks

Within aproximately 15 minutes, all Microsoft Windows background process should immediately execute all background idle tasks to completion, including tasks such as the Windows prefetcher, (which is a memory managment applicaiton used by Windows XP to help applicaitons to load faster).

To lock your PC in Windows XP you can press CTRL+ALT+Del and select lock workstation.

Another way is to have a screen saver that will lock the PC whne it launches.

This posting looks at creating a desktop shortcut that will lock your Windows XP PC.

First you need to right click on your desktop and chose new, then select Shortcut.

This will launch a shortcut wizard.

The first box will say:

This wizard helps you create shortcuts to local or
network programs, files, folders, computers, or Internet
addesses.

Below this you will see:

Type the location of the item:

Here is were you either type or copy the commands below:

rundll32.exe user32.dll, LockWorkStation

Click on Next.

For the name for this shortcut, enter in Lock workstation the nclick on Finish.

Now when you double click on this shortcut your pc will lock requiring you to enter in a password to use the Windows XP PC again.

This is a great way to lock your PC in an event if you have to walk away from your PC and you have sensitive information on the screen.