I client of mine asked the same thing with the same product about a month ago. Although I knashed my teeth and stomped my feet about using it, I installed it and it worked fine. Go figure, but a firewall is a firewall, anyway you look at it. Ancient, but it worked.....
Convince them to trust the MS firewall, and convince them to switch to AVG.
If not - no I cannot see any issues - provided the product is still valid. Usually Symantec allows you to use the product for a set period of time.... I tried reinstalling Norton 03 on a client's system (same deal as you they wanted it) and even after wiping the registry clean I couldn't install it.
I worked around it by setting their system clock back to Novemeber 2003.