Can a website see what I have running on my computer?

if your computer is up to date then most likely they will only get your browsers id which for the most part puts out something along the lines of " Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405 " .....now they will get an ip but shouldnt give out much of any detail on location.........now if you get a virus or something then a person can get plenty of information on you which is way anti-malware programs are recomended