Floating Point exceptions are almost always caused by a virus infection on your computer.
You need to scan your system ASAP, please find information and free scanners here:
http://steampowered.com/forums/showthread.php?threadid=76462
You should also scan your system for spyware as well since very often these problems happen together.
NOTE: Removing virus infections from your computer may damage files so be prepared to do a re-install of Steam or even of the operating system if you remove the virus but you still get errors.