ALWAYS DO A SYSTEM BACKUP BEFORE REMOVING/CHANGING SYSTEMS SUPPORT PROGRAMS
Download windows install cleanup (on MS site and others). Run it and delete any installer files for MS net framework.
Remove net framework with add/remove programs if shown in the list. If not in list you probably have had a partial or aborted update in the past.
Then download netfx_setupverifier (google for it) and you can determine the latest net framework version that is installed.
Then download dotnetfx_cleanup tool (google for that too).
Run the cleanup tool and delete any version no. higher than the highest good version shown in the netfx_setupverifier tool.
You should be able to download and install the latest version of net framework (believe it is 3.5 sp1).
Check add/remove programs after installation to prove 3.5 sp1 was installed and registered.
Run a windows update to get any updates to 3.5 sp1.