Windows 7 is going to launch Thursday 22-October-2009 and I think it would be better Idea to prepare your PC for windows 7 rather than watching the installation progress bar for hours in the installation process. Here given is a set of instructions “How you can upgrade your windows XP and VISTA to Windows 7”;
Before you buy an upgrade disc, though, you'll want to ensure your system meets the minimum specs for windows 7. Here they are in table form, stylishly cribbed from Wikipedia's Windows 7 ...