sometimes when I try to start minecraft 1.5.2 I press play, after that the mojang screen appears and minecraft crashes with the following crash report.

The weird thing is that when you scroll down it says: is modded: very likely, jar signature is invalid.

I swear I didn't mod anything so I just went to %appdata%/.minecraft/versions and deleted the 1.5.2 folder. after I restarted the launcher and it redownloaded 1.5.2 everything worked fine for a few days. untill a few moments ago, It crashed again saying 1.5.2 was modded.

once again, I didn't use optifine, mcpatcher, feed the beast or any other mod.