Q: my ibook g4 hangs on blue screen except in safe mode

You could try & start the computer from the install-restore system disc

to see about repairing the hard disk drive, if a verify says there's issues.

Since you don't say what model iBook, or OS X system running in it,

the Blue screen instructions vary by OS X and vintage; the Gray screen

instructions are more generic. System damage can be the cause of a

Blue screen on startup. Sometimes limited to one user account, but

could be large enough an issue to consider more drastic actions.

There are Support articles, I found one that says a few things to try, but

it was about Gray screen during startup, not Blue screen issues. Since

it starts in SafeBoot, it may be repairable. Depends on the cause.

Usually incompatible or damaged login startup files can make Blue screen

and SafeBoot starts in a reduced system mode, and does a self-check...

A second user account may start up normally, if there is no similar defect

in the file system, and not need to run in SafeBoot.

Gray screen appears during startup:

http://support.apple.com/kb/TS2570

Blue screen startup (later OS X, basic concept the same)

http://support.apple.com/kb/PH11043

The fact it starts past the Blue screen in SafeBoot is a good thing. You

likely could look into the Console or other system logs to see details

that present themselves by Time and Date of when it happened...

If you have a complete backup of the system, or a clone on external HDD,

then you may have to test the hard drive, the software, and also consider

the need to perform a secure erase (one to seven pass overwrite zeros)

with Disk Utility, then reformat, partition map for the OS X, and then install.

The same utillity, accessible on booted install disc, can do many things,

so you may not need to reinstall; since the system boots, it may be saved.

Unless there is a low-level defect in the format or sectors of the hard drive.

A secure erase with overwrite of zeros can help resolve that issue. Yet if

the hard drive is old and seem more than 5 years of use, it could be failing.

Good luck!