Solution 1: As described in the eclipse wiki link, please remove the mylyn update.
Solution 2: This may work
Problem: Eclipse 3.5 software update does not work. You see errors such as "No repository information found".
1) Help -> Install New Software -> Available Software Sites
2) Select all and then export it as bookmarks.xml
3) Now remove all the urls from the dialog box.
4) In a text editor of your choice, go into bookmarks.xml and add a "/" at the end of all the urls.
5) Now in the eclipse dialog, import this "bookmarks.xml".
6) Enable all the urls. (You need to click on the Enable button while having all the sites selected).
That's it. Now you will be able to get away from the errors.
Caveat: Every time you add in a new software site to update/install, remember to add in a "/" at the end of the url.