As Mike said In IIS that is CLR version, ( Thanks for letting me know Mike).
I can suggest a long shot . Why donot you obtain the HTTP_USER_AGENT from servervariables and check the .NET VERSION... this will only realy only the available versions of .net but may help you.
check this snippet
http://www.tamilcodes.com/tamilcodes/document.aspx?content=Snippets&id=43
my USER AGENT STRING IS LIKE THIS
HTTP_USER_AGENT : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; .NET CLR 1.1.4322)
I am assuming the machine you are testing is your local machine, IF it is the hosting provider , then you have to ask them.
If it is your machine then finally you can download 3.5 sp1 and install it