I have .Net framework 2.0 installled on my server, and websites are currently running that are developed in .net 2.,
my clinets wants .net framework 3.5 or if i installed .net framewrok 3.5 in my server, there is any problems to my previous web application that was hosted using .net framework 2.0
if yes ,how can i installed .net framewrok 3.5
Don't worry...just install it. .net 3.5 is incremental and works on top of 2.0. Any existing websites will reference the 2.0 binaries and will still work.
Also, I'd recommend you install .net 3.5 sp1 while you're at it. sp1 has been out for some time now, and many devs may like the new features.