why are my HD partitions hidden?

hi,

what tools have you tried,
try Disk manager under windows?
you could try gparted from a live bood cd, or partition manager

Thank you for your response.
Actually I have tried Acronis Disk Director and I finally could restore only one partition by re-sizing it's volume, and now it appears in My Computer.
For the another drive, it seemed like I had converted it to a primary drive mistakenly.
My question now, if I converted the hidden primary partition into a logical one, will this erase all the data in this drive or not?
Thanks.

converting a primary partition into a logical one is impossible without data lose. You could try to back up your information on a disk or external hard drive. If your Windows 7 OS is on this primary partition you might want to take one of your other primary partitions download linux or another free OS. After you boot from the Linux OS you can back up all your data on the partition you want to convert to an external hard drive. this link can help with the converting process.

http://www.partition-tool.com/easeus-partition-manager/convert-primary-to-logical.htm

You should check for the partitioned parts in Disk Manager and if its not there try re-booting the computer & maybe partitioning the harddrive once more.