hi there

one of these two solution should slove the problem ***HOPEFULY***
1-try to create a new object then go to four veiw mode then backto prespective veiw then undo
if this fixed the problem, then you have to upgrade you display driver to the latest ATI driver

2-try to rest your preferences you can do that by
windows==> setting and preferences ==> preference then in the menu rest it to dfault settings

hope that will help