It seems like the Windows OS is not loaded correctly which is likely the case as it seems XP was installed and then upgraded to Vista (a dirty install which has given you driver issues and other problems).It could also be that the hard drive is failing. More than likely the first symptom.

What is the model of your Dell?

First run the full F12 system diagnostics:

http://dellwindowsreinstallationguide.com/f12-preboot-diagnostics/

Then follow my A Clean Install of Windows:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/

Load Windows Vista or better Windows 7 (requires a new license) directly after a secure wipe of DBAN:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/

Note you can also securely wipe with DBAN and install Windows 10 directly on it. Windows 10 is currently available as a Technical Preview and designed for testing purposes only but seems to be very stable:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows-10/

Thank you for the reply. However, my O/S was always Vista. [The Dell rep said this problem first occurred in computers with XP, and was then also found with Vista.] My operating system is Inspiron 518. The Rep ran the Dell Diagnostics, and then did a complete clean reinstall. Do you have any additional suggestions what the problem might be?

Thank you!

Paul 

Do a complete clean install after a secure wipe with DBAN:

http://dellwindowsreinstallationguide.com/securely-wipe-your-hard-drive-with-dariks-boot-and-nuke-dban/

Use the WSUS Offline Update Program to download and install all of Windows Vista Updates:

http://dellwindowsreinstallationguide.com/wsus-offline-update/

Ensure these updates are installed before the system drivers, I suspect some conflict between the drivers and the service packs are causing the issues. Installing the updates before the drivers may resolve the issue. You may have a look at A Clean install of Windows for more details:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/