[Eru]

Try closing down all programs before playing fiesta. This includes 'power toys' in the system tray, and press ctrl+alt+del and close any non-critical processes there. Also turn aero off and use windows standard/classic/whatever it's called on NT6, and turn any desktop effects off.

See if that helps.

Edit:

Also run regular (I do it daily) virus / spyware / adware / malware scans too to catch any naughty programs that are leeching your system resources.

Edit2:

Also take a look at the resources (?) tab in task manager to see what's using the most memory / CPU time

yep...I've been doing all that since day one