Is Ubuntu 11.04 a 64 bit or 32 bit system ?

It's available in both versions. You can get 64 bit if your machine supports it or you can go for 32 bit. Ubuntu's latest version is 13, not 11. So i suggest using the latest version.

All recent versions (including 11.04) were released for both 32 bit and 64 bit architectures.

11.04 is obsolete. A laptop that's "only" 5 years old should have no problem running the latest version of Ubuntu: 13.04. Download either the 32 bit or 64 bit version, as appropriate for your computer.

There are 2 versions for Ubuntu 11.04, 32 & 64 bit.

In your case, You should use a 32 bit version for a 32 bit computer.

http://old-releases.ubuntu.com/releases/natty/

It can be either, depending on what you downloaded and installed:

http://www.ubuntu.com/download/desktop

I'm not sure if the 32-bit has that 4GB memory limitation like Windows but if you have 4GB or more of RAM then I would suggest that you install the 64-bit. I think you need to do a clean install if you have to change from one version to the other.

Ubuntu offers and supports versions in both architectures.