That... is a slow computer. Tried windowed mode? That should make the game render less stuff, which will improve performance. If you are desperate, you can boot with almost nothing running (don't start any background programs, indexers, firewalls or virus searchers, dump steam, torrent, avast, explorer itself), but remember to don't do anything else internet-related at that point, as viruses
Minecraft runs on Java, which is generally much heavier than other programming languages - it talks to a virtual machine that talks to the hardware, instead of going straight down - so you'll probably not get it to run properly on anything slow. It also loads a shitton of objects at once, so it'll eat up a lot of your ram.
Though, if you have something comparable to what's specified here: http://www.minecraftwiki.net/wiki/Hardware_performance
, and you still get a lot of lag, there might be a problem somewhere else. Do you have an integrated video card? That'll make it pretty difficult for you to get much performance out of anything.