Which keys? It could be a hardware issue. It's not malware, as no Mac OS X malware in the wild produces those symptoms.Just started, certain keys do not respond.
If permissions are repaired all is well for a few days.
Tried another keyboard, same problem after a day or two of use.
Perhaps have downloaded Malware or ?
Been swapping out keyboards with other Macs in the office.
Keys that do not respond seem to vary.
PPC Mac Mini, upgraded from 10.3.9 (original installed OS) to 10.4.11 about 4 months ago but problem with keyboard just started last week.
Try resetting the PMU.
http://support.apple.com/kb/HT2183