Customers trying to upgrade their recently purchased XP systems to Vista by cashing in their Express Upgrade coupons cannot do so due to massive demand for the update.

This morning Dell's and Toshiba's Vista-upgrade websites both displayed error messages and Moduslink, which was subcontracted by Microsoft to deal with the Vista Upgrade Express program (and directly handles all Mesh, Evesham, Acer and other smaller manufacturer's upgrades), was also unreachable.

Speaking about Moduslink to a disgruntled customer in our forums, a Mesh spokesperson said: "Since the release of Vista, the telephone number has been affected by the massive take-up. Yesterday the line went down completely. The lines will be upgraded and the number increased but this will take a day or so."

As of 1pm GMT today, the Moduslink telephone number (00800 666 44 666 – the double zero is not a mistake, it is a number based in Holland) is still shut, although the website appears to work.

Dell and Evesham were contacted about the programme, but no-one was available to comment.