With the advent of Microsoft's Vista, many people are considering upgrading their CPU.
Its requirement of a minimum 800Mhz processor will force those with anything slower to upgrade, or stick with XP.
Swapping out CPU's isn't that difficult.
Where the problem comes is finding a chip of the right socket type for the computer's motherboard.
Though Socket A isn't so terribly old, it's increasingly difficult to find one new.
If you need something from a little farther back, like a Socket 7, forget it.
You just won't find it.
At least not new.
However, there are plenty on the market for every socket type known, if you're willing to buy a "pull".
Pulls are CPU's salvaged from used computers.
They are carefully removed, cleaned, and tested (at least those sold by reputable dealers).
Processors, being solid state parts, far outlast the mechanical parts of a PC.
Some pulls are rescued from store returns.
These were units that for whatever reason failed to sell.
Such pulls are as good as new, and may be advertised as a special value.
The best aspect of buying a pull is the price.
I've seen them sell for as little as $4, though the standard price range is from $7 to $30.
Web searches done recently have revealed Vista speed processors in the $18 range.
There are many websites selling pulls, so check several to get the best bargain.
Most give a short guarantee, usually on an "exchange only" basis.
Use bulletin boards and newsletters you trust for references.