Ideal hard drive size for Time Machine

My HD is 320GB
I'm planning on using my 160GB hard drive as Time Machine drive

That's only enough if you have 80 GB of data or less needing to be backed up on that 320 GB drive. The general rule of thumb is that the Time Machine drive should be 2-3 times larger than whatever you're backing up.

By the way, if I want to use a a partition from a hard drive(i.e a hard drive with several partitions), it's possible?

What's possible? You could back up to one partition, if that's what you're wondering, but don't even think about backing up data from one partition to another. See my [Mac Backup Guide|http://www.reedcorner.net/thomas/guides/backups> for more information.

Does the Time Machine data will be accessible on Mac OS X and Windows OS?

No, only on Mac OS X, and you should never access it directly at that. Only through the Time Machine interface.

(I'm concern about virus in Windows that may harm that data and about accident data change during connecting the hard drive to any OS)

The only way to be sure to prevent viruses in Windows from damaging the data is to either not get viruses (use a virus checker on your Windows machine) or don't connect the drive to the Windows machine. As to accidental modification... just never mess with the backup data. Ever.

In addition to the specifics provided by Thomas, you might want to review these:

Time Machine Tutorial
Time Machine 101
How to back up and restore your files
Time Machine Features
Apple - Support - Mac OSX v10.5 Leopard Time Machine

and perhaps browse the Time Machine - Frequently Asked Questions *User Tip* at the top of this forum.

As long as it's large enough, partitioned and formatted right, yes. See #5 in the Frequently Asked Questions *User Tip,* also at the top of this forum.

But note that while TM can be made to back up other partitions on the same disk, it isn't a good idea -- when the drive fails (and they all do, sooner or later), you risk losing both copies. So either don't put anything important there, or back it up some other way, such as archiving to CDs/DVDs.

Well, yes, if the HD fails, you may lose some or all of what's on it.

That's kinda the main point of a backup -- having (at least) two copies of everything important, in (at least) two different places.

If your internal HD fails, you have the backup. If the backup drive fails, you have the internal.

So if there's important data in another partition of the same HD as your backups, you really should have it backed-up somewhere else (like CDs/DVDs).