Unfortunataly there is no solution to this problem exept to keep steam on at all times. Consider clearing or De-Fragmentating your Drive

Your situation is not normal. Perhaps you installed mods and its detecting them, trying to fix, but failing. From within steam, delete TF2, then exit steam and delete the TF2 folder. Start steam again, downlaod TF2 again (long process). After this you have a clean version and it shouldn't keep validating, ever again. Once is fine unless you notice it misbehaving. If this clean version is still doing that check any other game to see if that also validates every time. If so, time to reinstall steam. If after all that it still does it, your computer has a virus or is extremely misconfigured.
Good luck.

it maneged to fix itself and now my freind has that problem.thank you for you help if it happens again i will try above.

if i do this will all my stuff in tf2 be lost?

Originally posted by motoracer009:

if i do this will all my stuff in tf2 be lost?

Nopers!

Originally posted by motoracer009:

if i do this will all my stuff in tf2 be lost?

idk why everyone thinks that. it says it wont when you go to tf2 in the steam store. no offence, but some people need to read. if its a game you want to play, you should check if you'll lose your items when you unistall tf2.