.Net Framework Requirements

.Net Framework 1.1 or later is required:

• Windows Vista and Windows 7- .Net Framework is built-in (always present).
• Windows older than Vista - if you updated Windows over time, .Net Framework is present (at least version 1.1). If not, download it free from Microsoft. Here are links for some versions - although you only need to install one of them:
http://www.microsoft.com/downloads/details.aspx?FamilyID=262d25e3-f589-4842-8157-034d1e7cf3a3&displaylang=en (version 1.1)
http://www.microsoft.com/downloads/details.aspx?familyid=0856EACB-4362-4B0D-8EDD-AAB15C5E04F5&displaylang=en (version 2.0)
http://www.microsoft.com/downloads/details.aspx?familyid=10CC340B-F857-4A14-83F5-25634C3BF043&displaylang=en  (version 3.0)
a.s.o.

How do I know what version is installed on my machine?
Go Control Panel > Add or Remove Programs, and look for "Microsoft .Net Framework"; it has the version number appended.