HI there, welcome, you've posted in the right place so don't worry.
Computers can be built for as little as £100, but i'd argue that you get what you pay for (well, under £1000 anyway).
So tell us more ... what is your budget really.
What do you have already (keyboard, mouse, speakers, monitor etc).
Where is it going to be? (Living room, bedroom etc).
Do you want it to be portable? Cool-looking? Quiet? Energy efficient?
When you say multimedia, do you mean playing DVDs and mp3s, or streaming AV files around to other components.
Is the operating system important to you? (Some people are only familiar with windows).
Will you be wanting to edit videos, or play games?
Will you be wanting to connect other hardware / gadgets to it (iPods, cameras etc)?
I'll try to help if you can give me more info.