Possibly. Basically there are several ways to something close to what you want but none seem perfect...
1) Connect the PC to the TV
Could do this using a VGA to TV adaptor or a new graphics card that has a TV output. Anything displayed on the PC would be displayed on the TV but you need to find a way to remote contol the PC from the TV (eg to change camera). You wouldn't have each camera on a different TV channel.
Your PC sounds a bit old and might not cope with a new graphics card very well so an external adaptor might be the answer if you can figure out how to control the PC from a distance.
This sort of device converts VGA to AV..
http://www.maplin.co.uk/Module.aspx?ModuleNo=223833&C=Maplin&U=SearchTop&T=PC%20...Then one of these converts AV to UHF (eg TV aerial) for distribution around the house..
http://www.maplin.co.uk/Module.aspx?ModuleNo=33050&C=Maplin&U=SearchTop&T=UHF%20...2) Connect cameras to both the PC and the TV.
You could possibly do this with four of these and some cables..
http://www.maplin.co.uk/Module.aspx?ModuleNo=33050&C=Maplin&U=SearchTop&T=UHF%20.....but it might not work or work well. Technically you shouldn't connect a camera to two devices (PC and TV) without using an amplifier. Amplifiers are likely to be another £29 each so potentiall we're talking a total of about £240 for this approach unless a multi channel amp can be found at a good price. Picture quality might not be that great.
Hpwever this route would get each camera on it's own TV channel but not the "four at once". That would still only be available on the PC.
3) Use a box like this.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=370049056776&ssPageNam...This does a similar job to the video camera card in your PC. It allows you to select each camera or display four in one. It has two video outputs. One could be connected to the PC and one to the TV system (via a UHF modulator if necessary).
Still two problems: The PC could only display the output of this box so it's functionality is reduced. You would also need an IR Link device to get the remote to work from where the TV is.
Basically it all possible just messy and hard to explain.
Got a local CCTV expert?