The new and improved version of Parallels Desktop for Mac shipped this week, but there's a short-term upgrade obstacle for existing international users of the virtualisation software.

Some particular improvements in the new version include Mac OS X 10.5 Leopard and Vista support; the capacity to migrate settings from old PCs to virtual machines; USB 2.0 support and the much-lauded Coherence mode, which allows users to employ Windows applications on the Mac OS X desktop.

Existing users were asked to apply their existing activation code to the updated software, which was made available for download — but UK users have been frustrated, as the system did not seem to work.

Macworld UK reader Oliver Saunders wrote: "Our activation code does work on the newest much-heralded version. There does not seem to be a logical reason why we should be treated this way or at least they have not made it public."

Saunders and other affected users need not worry, as the situation will soon be resolved, reassured a representative of the software's UK distributor, AvantQuest.

"UK users who have purchased a retail boxed version of Parallels (ifrom Amazon or the Apple store) do not have to download the update from the Parallels website. They should set Parallels to automatically check for updates (or go to Help>Check for Updates) and the update will be pushed down that way, free of charge.

The representative warned that the build versions of the software currently being made available on the Parallels website are "not compatible with UK retail builds (well, the activation keys are not) and should not be used".

She explained that the builds being made available online through the US site are different to builds made available in the UK. UK builds are only made available when they are finished products "therefore builds are kept separate".

The representative urged patience among users, as the situation will be resolved in the next few days.

"Today and over the weekend, all builds, UK, US, Beta, and so on, should all be pushed up to build 3186. This may take time as Parallels only have the one server to get this out."

The news means UK users of the software should be fully up to date by early next week, but they must ensure their copy of Parallels is set to automatically check for updates.

"We are obviously keen to ensure that all the UK users get their upgrades," the AvanQuest representative added.