When I leave my computer on but don’t use it for about 15 minutes it goes onto wallpaper only – all the icons on the desktop and task bar and tray disappear and the Start button disappears so the only way to exit is via the Power button and start up again.

I don't know if it is related but before this I had noticed that that the computer seemed to be slower than it used to be e.g the time it takes from inserting my password into the user account to the personalised settings finishing the loading up process and also generally moving from one thing to another ( It takes a long time between selection of an item to it appearing on screen ) and many instances of programme not responding.

I have Windows Firewall Antivar, Windows Defender, Antivar Antivir Personal free anti virus and PC Tools Free Spyware Doctor.

Total disc space is 74.52; the free space is 49.55 and used space 24.97

My Computer Details are

HP Pavilion zd8227EA Notebook PC
Operating System Windows XP Home
Memory 512 RAM
Speed 2992 MHz
Processor Intel Pentium 4 CPU 3.00 GHz
Date of Purchase August 2005

Hi keith,

It sounds as though explorer.exe is closing. This is the Windows shell or GUI (Graphical User Interface) which shows your desktop and all the programs you have running.

To check if it is explorer.exe, the next time your desktop disappears please press CTRL+ALT+DEL > Click Task Manager > Select the Applications tab.
Click New Task - down the bottom > in open type: c:\windows\explorer.exe > Click OK

Does everything come back up after you do this? If so then explorer closing is what your problem is. The fun part is troubleshooting what is causing it to stop in the first place.
Please check your screensaver options? Is it set to switch on after 15 minutes? Also check your power options in control panel. Is it set to turn off the monitor and/or hard drives after 15 minutes?

Please let me know what you find in your next post.

Followed your advice - Problem sorted - Screensaver/Power Options needed changing.