Need a little more info here, Pavitar...
- What is the host OS?
- What version of Ubuntu are you attempting to install?
- Have you attached the CD or ISO to the Virtual CD Drive? (So it can boot and install to the virtual HDD)?
Unless you have monster amounts of RAM, 50% is a BAD idea. I assign 50% of my RAM on a server that has 16GB RAM, but I wouldn't even attempt to run a VM on a machine with only 1GB RAM.
I definitely recommend a RAM upgrade if you expect to get any kind of decent performance out of your VM - and for that matter, your host (since 50% of your 1GB is allocated to the VM, your host only gets 512MB).
You may also want to check out http://ubuntu.stackexchange.com/ for Ubuntu-Specific help.