Wireless repeater/ range extender

Out house is a relatively small house (1600 SqFt) but our router won't reach to the far end of the house, or underneath the house with good signal.
Our router is from the ISP but we have a spare router (if I can find It) but could I set up this router to be a extender?
We have CAT5 in the walls run to each room, but the ISP didn't connect to that, they dropped a wire from the attic and bypassed that system, could I run an ethernet cable from the LAN port on the back of the router to the wall system and hook up the old router in another room?

 

What kind of spare router is it?

I use wrt54g for my repeater since they are Linux based and there is 3rd party software like ddwrt which enables a ton of custom settings like being able to use as a repeater.

 

If the house is prewired for CAT5, set the second router up as an AP (turn off DHCP) and use the house wiring to connect it to the main one.

So, ISP Router (any normal port, usually 1-4) ---> CAT5 Wall Jack ---> Jump to another CAT5 jack at demark (hopefully they are labeled on the punch block so you can just plug that jack into another to jump it to another room) ---> Other Jack to second router (Normal port NOT internet port) in AP mode

EDIT: IF these wall jacks all go to a switch, you won't have to mess with jumping wiring at all. Just plug the ISP router into one and the second router into another.

 

I believe it's a linksys, It isn't very old and IIRC it's a dull band router, I can't seem to find any info on the ISP router.

Ok, I'll ask my Father how the wall jacks are done, he set it all up when he built the house.

Thanks for the info guys.