PowerPC G4 became very slow, please help!!!

I have an old PowerPC G4, OSX 10.4.11, Dual 1.25Ghz with 1GB DDR SDRAM.

I haven't used this computer for a while, but it has always worked normally. A couple of days ago I started copying all my personal documents, folders, pictures, videos and music inside one folder so I could transfer to a hard drive, then to my new computer.

After I finished transfering everything I wanted inside that one folder, the computer became VERY, I mean VERY slow, like if you click OK or CLOSE, the rainbow wheel starts turning for like two minutes. Anything I do brings back that wheel. It is almost impossible now to plug in my external hard drive and transfer the data. I really need that one folder but don't know what to do now. Any ideas?