This is where I really, really need an expert opinion. Awhile back the power went off in out house while my computer was running. Though I cannot be sure of this, It seems like, since that time, the overall operation of my system has not been the same.

Vista takes longer to load, protocols and programs don't work as good as they did, and so on. My question is: Can a power failure do damage to a computer that is unseen? I mean, can it affect parts or components of the motherboard or overall system that are irreversible and in such a way that it will affect not only high-end performance but normal daily use?

The reason I ask, is that I'm building a new system and I want to make sure it's protected in the future.

For the new system I'd include a UPS along with basic surge protectors.

As to your past issues I doubt very much that a power issue would affect performance. In general a power issue would have rendered much more obvious issues such as not starting or BIOS errors.

You can test your hardware. Run MemTest. Run your HDD diagnostic. Run basic Vista cleaning: defrag; disk clean; check disk; etc.

If you have a Restore Point prior to the problem try a Restore.