Cheap Windows 8 upgrade may require your Windows 7 key

People who bought a Windows 7 PC on or after June 2 can upgrade to Windows 8 Pro for just $14.99, but they may have to supply their Windows 7 product ID.

Microsoft kicked off registration for the $14.99 deal in August. Anyone who bought or will buy a new Windows 7 PC between June 2 of this year and January 31 of next year can upgrade to Windows 8 Pro for that special price. Shelling out the $14.99 lets you download a copy of the Windows 8 Upgrade Assistant, which helps you install the new operating system.

The deal cuts $25 off the standard Windows 8 Pro upgrade, which costs $39.99 to download for Windows XP and Vista users and Windows 7 users who purchased their PCs before June 2. People who want an actual installation disc have to cough up $69.99.

The $14.99 upgrade process typically asks you to enter the name and manufacturer of your Windows 7 PC, and the date you bought it. You'd then receive one of two notices, either a Success message telling you that you qualified, or a Sorry message breaking the news that you didn't qualify.

But in my experience, that process seemed a bit buggy.

I tried to register for the $14.99 upgrade last week with three different PCs, all from Lenovo and all purchased after June 2. One PC qualified for the upgrade, but two did not, with no explanation why. I tried the process a few times with the two failed PCs, but each time I came up empty.

I again tried the process on Tuesday and this time discovered that Microsoft now asked me for my Windows 7 product ID as additional proof. After entering the ID -- success. I now qualified for the upgrade.

Based on various online comments, other recent Windows 7 PC buyers were bumping into the same initial problem of being rejected for the offer.

However, the requirement for the Windows 7 ID seems to be random.

Some people told me that they were asked for their Windows 7 key in the past and pointed out that this is not a recent change. But others have said they've never been asked for the key, not even in recent attempts. I contacted Microsoft to ask about this inconsistency, and a spokesperson e-mailed me the following statement:

The Windows Upgrade Offer process will dynamically adjust the registration process based on a variety of factors that take into account individual, local market, and global circumstances. Some users are required to provide additional purchase details online or offline in order to validate their registration. If eligibility cannot be determined using the online process, customers may need to contact customer support and provide proof of purchase to support.

So it seems to be a hit or miss process as to whether you'll have to supply your Windows 7 product key. But in my experience, providing the key was the only way I could qualify for the upgrade.

The offer limits you to five PCs, and you can install the Windows 8 upgrade on any PC, not necessarily the one with the product key that you use to qualify. The $14.99 and $39.99 deals both end January 31. So those of you want to upgrade to Windows 8 Pro should take advantage of those prices while they last.

Updated at 3:30 a.m. PT December 20 with clarification and response from Microsoft.