Yes It is Directly possible to upgrade from 5.1 to 7.0.... for the more info refer as below
With the release of WebMatrix it is now possible, because WebMatrix itself brings instalation of IIS Developer Express which is based on latest IIS7x codebase and can be installed on any supported Windows operating system:
- Windows 7
- Windows Vista SP1+
- Windows XP SP3+
- Windows Server 2003 SP2+
- Windows Server 2008
- Windows Server 2008 R2
IIS Developer Express supports all of the latest IIS7 modules, including URL Rewrite, FastCGI, etc. It support Classic ASP, PHP and of course ASP.NET development, including integrated pipeline.
How to install on Windows XP
Refer :
http://mkdot.net/blogs/latek/archive/2010/07/23/iis7-on-windows-xp.aspx