Platinum bolts are the "special currency" found in hidden locations on each planet, and used to do super-special upgrades to your weapons. You buy those upgrades at secret upgrade locations.

Finding the Platinum Bolts
Oozla: Go up to the left in the room you get the tractor beam in.
Endako: Go straight to a ladder after the helicopter area.
Notak: leave ship, short bridge, go left, go behind building.
Notak: Take out a sign in the second mall area, in the far left side.
Notak: Dynamo the door on the docks
Siberius: drag cylindar up elevator and over to ledge
Tabora: Glide down canyon
Tabora: At far edge of desert

Using the Platinum Bolts
Endako: Ride the rails to his shop.
Tabora: Just north and east of the ship, in the desert

The Weapons Upgrades
acid mod for stars - 2p
shock mod for vaporizer - 1p
acid mod for blitz - 2p
shock mod for heavy lancer - 3p
shock mod for megaturret - 2p

You can only platinum-upgrade the evolved versions of weapons.

Ratchet and Clank 2 Going Commando Walkthrough