If you are running a PC or Laptop with XP already on it then I suggest your DONT upgrade to Vista.
This is not for anti-Vista reasons as I have a PC with Vista on it and it is fine.
But an upgrade copy of Vista is going to cost you a lot of money (�50 or �60) and if your computer is a few years old it will not be powerful enough to run it.
Vista really needs a decent modern CPU, with 2Gb of memory, and if your PC is less than that it will just crawl along.
Also the upgrade process can go wrong (it has gone wrong twice when I tried to do it and I know a lot about computers) and you could finish up with a completely unusable system.
If you REALLY want Vista then consider buying a new System unit to replace your existing one (assuming you have a PC rather than a laptop).
Dell recently were selling sysytem units for �160 with Vista on them and this is a far better use of your money than an upgrade.
See here for some computer deals (and other deals)
http://www.hotukdeals.com/