External hard drive missing some files!

Hello,

I recently plugged in my external hard disk to a windows xp desktop. After that, I plugged my external hdd back into my windows 7 laptop and my antivirus software, avira, came up with a few virus notifications suggesting that i move some of my files to quarantine, and i did so. Then i opened my external and found that some folders were missing, however when i checked the storage space it was still the same, like it never disappeared!

How can i recover them?

I have read that windows xp tends to have problems with large external hdd.

Additionally, I cant open my avira software as it has been corrupted!