Linux is an operating system in the same way that Windows is: It acts as an intermediary between your programs and (through the BIOS) the hardware of the system.
Ubuntu will almost certainly behave faster than your present system. However, you should know that your problem may not be a virus. Windows and Microsoft Office are notoriously bad housekeepers. Therefore, they leave temporary files around and don't always release memory back to the system when they are done. This builds up and can really compromise the performance of your system. This sort of thing can happen with any computer system, but Windows is particularly bad for it.
That being said, Linux and specifically Ubuntu are virtually drop-in replacements for Windows. You can access almost all of the same data files - Word files, Powerpoint, PDF, etc. The difference is that Ubuntu is freely available, known to be more secure out-of-the-box, and is updated more regularly.
Installing Ubuntu will probably speed up your system. You need to check the system requirements first:
http://www.easy-ubuntu-linux.com/ubuntu-system-requirements.html
The nice thing is that you can download Ubuntu and try it out. Instructions on how to take Ubuntu from downloading from the website to installing on your computer are here:
http://www.easy-ubuntu-linux.com/get-ubuntu.html
http://www.easy-ubuntu-linux.com/ubuntu-installation-606-1.html
The second link is a bit dated, but the process has not changed much at all.
Once you have it on CD, you can run it from the disk before you commit to installing it. This will give you some indication of how your computer will act with it. Note that running off the Live CD does not harm your data files because Ubuntu runs completely in RAM; it doesn't use the hard disk.