A huge number of users have been frozen out of the upgrade until an unknown date in April, but some were able to manually download SP1 as of last week via the firm’s Windows Update website.

However, a large number of complaints on the official Vista blog, over troubles with the service pack installation.

Microsoft blogger Brandon Le Blanc says that anyone, including customers with OEM copies of Vista, will be eligible for free SP1 installation support.

"You have a variety of options you can choose for support – all of which will NOT cost you any support fee."

Such a service is normally reserved only for purchasers who bought a retail copy of Vista, while those who already have the operating system pre-loaded on their machines are referred directly to the OEM or reseller and charged for the privledge.

According to the firm’s SP1 site, unlimited installation and compatibility support is available free of charge until 18 March next year.

Under the plan customers can email a special SP1 support address or use online chat.

If you are interested in reading the official Microsoft Vista Blog from Nick White (Product Manager at Microsoft working on Vista) you can read it here.