Does the C400 have an XP license ? Or do you have an non-OEM XP license that isn't in use on another PC ? Otherwise you will need to buy a license first. If you can't get a
trial version of Windows 7 you may still get a trial version of windows server 2003/2008 or any number of free OS's that you could use instead, TBH unless the C400 has more than 256MB of ram you might want to...