After several years of the thing working correctly, I have developed a problem as follows:
When I press the "sleep" button on my keyboard the computer appears to go into standby mode (this is what I want). But when I wake it by hitting a key or moving the mouse it hasn't been in standby but restarts by rebooting.

I have checked the power options in the control panel and I have asked it to do the right thing i.e. standby when the sleep button is pressed.

The only major thing that I have done this week (when the problem started) was to down load the latest version of Internet Explorer Version 7 - as prompted to do so by Microsoft.

Many thanks people....

What was the prompt from Microsoft?

There has been no update to Outlook Express that I know of and Microsoft does not prompt outside of Windows Update.

That being said you might want to run a System Restore to a point prior to this:

start > All Programs > Accessories > System Tools > System Restore

Ooops sorry I think I told you wrong. I got the latest version of Internet Explorer (my mistake - I've corrected the above now). I just opened the microsoft update stuff and was given a down load of a new version (the one with the tabs) that I didn't have before. I guess a roll back is an option but then I would go back to an earlier version of Internet Explorer and I kinda liked the tabs.....

IE7 was released last Tuesday as an Automatic Update which Microsoft listed as Critical: yes.

It is the current version for XP SP2 and Vista.

There have been some issues with IE7 affecting old ISP software. Especially insignificant diagnostic or manager applications.

Rather than going backwards I would suggest finding out what this conflict is as all Microsoft Updates in the future will be based on XP SP2 IE7 or else Vista.

I, myself, am about out the door for this evening but will look further when I return. Perhaps another of our members will also have ideas.

You might try going to start Run... msconfig | Startup

...and unchecking all the boxes except your anti-virus and firewall. You can always go back and recheck them. This just stops these applications from automatically starting. Especially any applications that deal with your ISP.