Computer owners upgrading to Windows 98 are encountering unexpected problems as many PCs need additional software before the new operating system can run properly. In addition, most existing PC owners risk losing valuable customer support from manufacturers if they switch to the new Microsoft operating system.

Leading PC makers are typically offering upgrades from Windows 95 to Windows 98 to customers who purchased PCs up to a couple of months prior to its release last month. However, users also need additional machine-specific software additions, known as patches, which may be distributed on separate CD-ROMs. These are necessary because Windows 98, as shipped by Microsoft, may not contain all the software to drive certain devices such as printers, CD-ROMs, and modems.

The nature of Microsoft's licence with European manufacturers prohibits them from selling retail versions of Windows 98. This means the majority of existing PC owners who want Windows 98 must purchase retail versions independently and download the device-specific patches from manufacturers' websites. Compaq, Dell, Gateway and Toshiba have warned users not to upgrade to Windows 98 until they have the patches for their machines.

But this has potentially serious implications for the technical support manufacturers supply to customers. Mr Michael Hughes, software product manager for Gateway in Ireland, said the Gateway would consider a Windows 98 upgrade purchased elsewhere "a third-party product. We are only licensed to sell Windows 98 to people who bought after April 1st," he said.

Despite Gateway assurances that "You're a customer for life once you buy a Gateway PC," Mr Hughes said free customer support "only applies to components we sold them". Despite assurances the company would offer support "to the best of our ability", a Gateway spokesman said: "We are not allowed to support software we don't supply."