partition unknown

I have a problem with one hard drive and one one USB stick.
Last week I could not access the USB stick and it was listed in disk management as "Healthy Partition unknown". Today the same thing happened to two of my hard drives. the first partition on one of them and the entire second hard drive are inaccessible through normal means (like explorer) and show up as "Healthy Partition unknown".
Luckily I got all my data back by using the "Recover my files" utility,
but the drives are otherwise useless.
Incidentally these were FAT32 partitions, but I do have an other drive with the same FAT32 format and is so far not effected.
Do I have virus? or what caused this?
The system is XP SP 2 all security updates.
AVG antivirus, Comodo firewall.
Everything else is working normally.