It depends whether or not you want the full version. If so (and I would recommend so!), then I would consider getting an OEM version (unboxed, etc).
The only drawback to this is that you will need to purchase a hard drive or processor, but when you consider that the cost of the s/w is only £55, then you may want to get a second hard drive as well!!
Check out click here anyway! Hope it helps :)