MSI RS482M4 Mobo not recognizes USB por...

I own a MSI RS482M4 with Athlon 64 3200 proccesor and Service Pack 3 Windows XP.

When system starts (only BIOS without any mass media storage connected) the MOBO recognizes USB media, but when SO boot begins automatically it ceases to recognize USB port, no matter what BIOS settings, new drivers installed or LiveUpdate or anything else.

I know USB are electrically ''alive'' because any thing USB with led that is connected gets on (got a fully iluminated led) but there is no recognition from XP SP3.

It's desperating because all the rest of the MOBO works OK, except that when I run task manager CPU appears with 100% usage.

I have done:

1.- Install and re - install SO XP SP2 and XP SP3
2.- Install and re install latest drives.
3.- Install and re install with diferent peripherics ( HDD IDE 80 Gb, HDD SATA 500 Gb, DVD/RW IDE, CD RW IDE).
4.- Flashing BIOS with latest BIOS scripts.

¿Any advanced suggestions?. Don't be afraid to be technical, PCs are my hobby.