Or buy a new HDD, put Windows 7 on it and just simply hook up all the other HDDs from your old PC to it.
Honestly as long as you don't format the partition/HDD where your files are, there are loads of ways to get your files from one PC to another. Especially make a backup of your
C:\Users\username folder as most probably all your saved games, settings for different software and also saved passwords used by your browser etc. are there. Last time I reformatted I just copied my
username folder over to the new OS and saved me so many hours.
On the subject of OSes. Windows 7 is the replacement of XP. It is the XP of the modern age basically. Vista was a failure ask anybody, NEVER go for Vista. I can't comment on Windows 8 as I haven't used it yet but from what I have seen it is aimed at Tablets mainly and Touch Screen devices. Here is a Thread made by @Sub on the subject of Windows 8:
http://forum.esforces.com/threads/153080-Windows-8
On the subject of Desktop vs Laptop the answer is: depends on the use. A desktop is generally more powerfull than a Laptop as it is stationary and is always powered from the mains. The problem is its more xpensive as the screen doesn't come with the PC usually + more high tech parts than a laptop. I suggest getting a Desktop for regular use and a Netbook/Android Tablet for traveling. Either way you go, Windows 7 should be the main OS unless you get a tablet in which case Android.
PS: I have also seen Android Netbooks recently. Can anyone comment on those? Debating getting one.