my computer has 512 mb ram..i am using xp..i want to install ubuntu 10.04 within this xp(without dual booting).I heard about virtual machine such as virtual box or vmware player that help us to do the same..so i want to ask which one will be good practice vmware or virtualbox..remember i have only 512 mb ram...

I have also doubt regarding these quereis..
1) can i install software of my choice in ubuntu after installation.
2) can i access internet by ubuntu
3) if i keep some data in ubuntu...will it be kept there also after machine shut down.

actually i did dual booting xp and ubuntu..i succeed but sometimes my computer get stucked due to low memory..so i want to try it and want to have the same performance/job ....

I'm not sure you understand the difference between dual booting and using a virtual machine (Or thats how it reads). Dualbooting is where your computer can start, in this case, either Windows XP or Ubuntu when it boots up.

A virtual machine is where you've started XP and you use Ubuntu inside this, using a virtual machine.

A virtual machine can use the internet, install and everything else. It will take a major performance blow though; I'm not even sure if it'll work at all on your computer. If it does it wont be any fun.