If you're planning on upgrading to the 64-bit version of Windows Vista , you might have problems getting hold of it. That's because the main way to upgrade to it is to buy the 32-bit version, then order a replacement CD from Microsoft.

The 64-bit version of the operating system is expected to become increasingly successful as users upgrade to PC with 64-bit processors.

You only have to send off for an extra disc if you buy the lower end Windows versions. Should you fork out for the £249 (upgrade) or £349 (full) Ultimate Edition of Vista, you'll get both the 32-bit and 64-bit discs in the box. Enterprise customers can also easily get the 64-bit edition.

You'll have to surrender your 32-bit key should you wish to upgrade online to the 64-bit version.