I just bought a psp ith 2.5 firmware on it, how do i get it to run iso's?

Upgrade to 2.80 (make sure it is 2.80, NOT 2.81): http://www.psp-hacks.com/category/49
Download the 2.80 downgrader and follow the instructions here: http://www.psp-spot.com/forums/280-generic...der-t33090.html
Check if you have a TA-082 (or above) before starting: http://www.modchipstore.com/support/images/ta82spot.jpg
If you have a TA-082 (or above) then you need to run the "TA082 IdStorage Changer" step; this is very important or you will brick your PSP.

is 2.8 a stable downgader, ive heard its not safe. Why dont you advise him to upgrade to 2.71... ive heard thats a more stable and tested downgrader.... im quite sure there is a reason you've told him/her to upgrade to 2.8 but why?

Theres is also a 2.60 downgrader. This is the one I used a long time ago when my psp was a 2.60.

The new 2.80 downgrader is stable though.

I would use either the 2.60 or the 2.71 downgrader regardless, but that could change as the stability of the 2.80 is better now.

*Also, its important to know what motherboard you have like adw stated. If you have a TA-082 you would probably have to use the 2.80 downgrader.

You're right in that the old 2.80 downgrader was not stable, but this is the new, improved 2.80 downgrader, which is meant to be easier and safer than the 2.50/2.60 and the 2.71 downgraders.
This new 2.80 downgrader also supports both TA-082 and non TA-082, with only one difference in the process.

2.71 is by far the safest version to downgrade to 1.5. I strongly recommend you downgrade from 2.71.