Is the battery still good? They have a habit of being a bit bloaty if left for too long. Though I guess if you booted it far enough to see what firmware you are on then it is fine.
Anyway that sounds like you might have a permanent flash capable 2000 which is pretty sweet if so and for many that is the PSP model of choice. 5.0M33-6 means you are on hacked firmware version 6 of M33's custom firmware based on the 5.0 firmware line of the PSP. It is old, and I am guessing it was hacked with chickhen exploits or something. You can do a lot with that firmware but I would definitely upgrade.
In the subsequent years there were more custom firmwares and the keys to the kingdom were found in the PS3s when they were hacked -- a lot of homebrew is now signed and runnable on stock firmwares, though you still want a hacked one for some homebrew and ease of running games.
retrofan_k's link will do nicely, in your case
http://wololo.net/installing-pro-cfw-on-a-psp-2000-slim/