i have dell optiplex gx620 with 2.2ghz processor
Your best best is probably using it to mine a CPU-friendly altcoin like Quark, Molecule, Atoms, Protoshares, etc...
As they are still relatively ok protocol to be mined on CPU only, it could be worth it. Also factor the power use in the equation as it could really offset the profit.
Eh no that machine can't mine anything really (I know I have several). Those old P4 are space heaters even at idle, if you were to 100% load the CPU you would burn more electricity than if you mined with a 7850.
About all the machine is good for is controlling some ASICs (which is what I use mine for).