Microsoft has apologied to customers for the lengthy delays they are suffering while waiting for their express upgrade Windows Vista disks.

In an exclusive interview with PCW, Robert Epstein, Microsoft's UK OEM group manager, blamed delays on "unprecedented demand".

He claimed that Microsoft was surprised by the number of Vista disks it was expected to deliver under the upgrade scheme, where people who bought an XP computer after October were offered 'free' Vista upgrades.

Under the scheme, customers can order an upgrade copy of Windows Vista, but many have criticsed the scheme after lengthy delays and being hit with unexpected charges, despite being led to believe by retailers that the software was free.

Epstein said Microsoft didn't know how many express upgrades it would have to deliver, although he admitted that the company began to get a sense of the scale of demand after the Christmas sales rush.

But he refused to answer questions on whether Moduslink - the biggest distributor of the upgrades and about whom there has been a flood of complaints - had done a good or bad job in distributing the DVDs. Instead, he said: "One of the reasons we're working with Moduslink is they're one of the manufacturer's of those [Vista] disks."

Responding to complaints by customers that they were led to believe the upgrade was free, yet have been charged for it, Epstein clarified that Microsoft does not take any royalty fees for the express upgrade.

It gets nothing from the postage and packaging costs, the administration fees retailers are charging or from participating computer manufacturers, he said.

When asked if he thought retail outlets such as PC World had been clear on charges and the use of the word 'free' to describe the express upgrade, Epstein said: "I have to say overall, yes I think the message was delivered clearly at retail in terms of how the scheme was operated."

PCW has received over 100 angry comments on our Test Bed blog (see Vista express upgrade on hold and Moduslink responds to Vista criticism) from readers struggling to get hold of their Vista express upgrade.

One reader complained: "I do feel that these companies are deliberately stalling the process so that not many, if any, register before the cut off date in March."

Other readers have been contacting trading standards to complain over the way the word 'free' was used to describe the upgrade, despite retailers charging a range of fees to provide the disk.

Moduslink has previously denied any wrong doing, claiming it was fully prepared for the scheme's response. "The volume is extremely high. The volume itself was anticipated but there were some specifically high spikes," said Christine Pothier, marketing and communications manager for Moduslink.

Dell, however, apologised for delay in sending out the disks in February.