You already have everything you need to run ASP.NET applications; Visual Studio 2008 has an inbuilt web server 'Casinni' that starts up and hosts your asp.net site when you compile and run from the development environment (Press F5)
If you have the professional version of XP you can install IIS 5 via Control Panel -> Add/Remove Windows Components and use that to host your website instead.
To configure vs.net to use IIS instead, In Solution Explorer, right-click the name of the Web site for which you want to specify a Web server, and then click Property Pages.
In the Property Pages dialog box, click the Start Options tab.
Under Server, click use custom server.
In the base URL box, type the URL that VS.NET should start when running the current Web site.