i'm a new user, so i dun really understand. so i shd check the items under 'THESE ITEMS ARE EITHER HARMFULL OR A SECURITY RISK WE STRONGLY RECOMMEND TO FIX THEM :', and click 'FIX CHECKED' in the hijackthis window?

Yes

is this irreversible?

No, hijackthis makes backups of what you fix, you can get to them by clicking config in hijackthis then clicking backups (both buttons), from the backup screen you can restore or delete the backups.

what about the items labeled 'THE FOLLOWING ITEMS ARE NOT NEEDED TO LOAD AT BOOTTIME FOR THE SYSTEM TO WORK PROPERLY :'? what shd i do with them?

These are items that load when windows starts up (you can see most of them in your taskbar nest to your system clock), for example this line here is MSN messenger “o4 - hkcu..\run: [msnmsgr] “c:\program files\msn messenger\msnmsgr.exe” /background”, fixing this will stop msn messenger showing when you start up your pc, you will still be able to use these programs by clicking on there icons or from the programs bar, they just won’t load at system start up, which means your computer will start up much much quicker then usal.

–lee