Instead of using the GTA downgrader, use the 2.80 Tiff Downgrader. It is faster and safer than the GTA version.
To update to 2.80, download the 2.80 update EBOOT.PBP and put it in X:/PSP/GAME/UPDATE/EBOOT.PBP where X is the drive your psp is on your computer. this is also called ms0:/ (ms0:/PSP/GAME/UPDATE/EBOOT.PBP)
or the Root of the memory stick(ms0:/)
You can get the 2.80 update here:
http://www.psp-hacks.com/file/663
Here's a tutorial:
http://www.psp-vault.com/Article800.psp
Then update to 3.03 OE-C by using the easy installer which can be found here:
http://www.psp-hacks.com/file/1043
Make sure to read the readme for help.
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