linux for a really shit computer

i have an older laptop that i'd like to use for web browsing (including youtube videos), downloading torrents and watching videos (including HD videos). i haven't used linux before but i'd like to because maybe i can do these things without excruciatingly slow program load and response times.

what distribution should i choose for speed? one that is a 'major' one. i tried google but i got a million different distros most of which didn't seem to have a big community or a lot of support. is lxde the best desktop manager to use?

i'm reading debian documentation atm. it's very confusing about partitioning. i've also looked up elsewhere and it mentioned gparted but said that it can ruin your windows installation. my laptop os is vista and i want to have both vista and debian on my system and i don't want to harm vista. what is the best course of action for doing this?