Can a Filemaker database (.fp5 file) created in a Mac be used in a Windows PC and vice versa?

Yep

Make sure you test it on Windows before deploying it. Some objects will be out of place, text may be different, and be careful because certain scripts are platform-specific.

Another no-brainer tip but worth mentioning..

It's mostly in the cosmetics.

Use a font that is common for both MAC and Windoze, such as Arial or Times and when designing the layoutouts give yourself a little more room around labels and fields, don't cut anything too close and you'll be alright.

Of course, stay away from anything that uses Applescript and be a little more leary of "external functions" in scripts until you test them. You can always get a cheap, decomissioned PC for $100 to test it out.

Thanks guys for the tips.