To answer your question of Can I use a laptop as a screen for a desktop
No you can't
the required specs for total war seem to be
OS - XP or Vista 32
Processor - Intel 2.4ghz or equivalent
RAM - 2GB (Vista) 1GB (XP)
GPU - 256MB Card, DirectX 9.0
HDD Space - 15GB
So it will need a 256mb graphics card, I would be surprised if a spare PC you have kicking about will have that sort of level card in it to be honest.
You need to know what your system specs are and check on the side of the games box if it meets the requirements for any games you buy before shelling out on them!