Jonathan Clarke offers some cautionary advice for those thinking of upgrading their PCs to Microsoft's Windows XP operating system

I was on holiday when it happened. A client using software that we have happily supported for years suddenly reported horrible errors as the system crashed in the middle of an everyday process. My colleagues helped them sort out the ensuing data problems and all was well for a day or two. Then it crashed again.

The only change to the system? The client had recently installed Microsoft's Windows XP.

At Whitespace, we installed XP last year on a few PCs to see how well it worked. We had mixed views about it. It boots quickly, which is nice. But some respected commercial packages, such as the Act! contact management software and the Visio diagramming tool would not run well.

Microsoft is upfront about this, explaining on its website that software run under one operating system may fail on another. "This may be especially true when migrating many older applications to Windows XP," it tells us.

There are helpful pages dedicated to telling you which pieces of software work on XP. Unfortunately, they list only recent versions which are either "designed for" or "compatible with" XP. They do not cover the older versions of any packages, including Microsoft's own Office applications like Word and Excel.

Other internet sites help here, (for XP software compatibility) gives far more information. So I know that users have found that in the Office 2000 Premium set of programs, only Word worked - Excel instantly exits. There are pages of comments about success and failure (Visio 4.0 is OK, Visio 5.0 fails).

But our client's software is a specialist insurance application and therefore will not appear in the standard compatibility lists from Microsoft and

At this point, a tidal wave of advice hits you. You can read up on how to configure a duplicate network to test your systems. You can tell XP to pretend it is an older operating system. You can run Microsoft's Application Verifier program to check for various known problems. You can run its QFixApp program, which knows about 195 problems and how to fix them.

Our client did the sensible thing - it asked us to investigate on its behalf. We pointed out that, although our daily charges are reasonable, we could not say how long it would take to fix the problem. Suddenly, the simple upgrade to XP could cost more than their computing budget for the year.

Is there an alternative, the client asked. Yes, we said. Windows 2000 has not caused the problem, so use that. Our client went back to that operating system and has had no errors since.

The only problem with this solution is that it ought not to have worked. XP is based on Windows 2000. Logically, software that works under 2000 should work under XP. This was not the case for our client. As mentioned above, lists XP compatibility problems with Office 2000, a Microsoft product written for Windows 2000. There is clearly something more complicated in all this.

There are people who really understand these issues, but they probably work for Microsoft. If we mortals try to work out whether or not a piece of software works we need to set aside a good few weeks to test everything.

Me? I'd rather be on holiday.

Jonathan Clarke is chief executive of Whitespace Software