Isit possible to prevent an OS (dual boot) to access/mount any other partition

your partition table seems to me to be almost correct, but I would think about an LVM (it could seems tricky, but I assure you that it will worth the hassle of understanding and using it).

The main reason is: (personal free-thinking :D) "I think that 20 GB are really a lot for a / partition (if you're not planning to put a public web server or something that could fill up fast your hard drive), and that 25 GB for home are really a few; but what can I do if someday I'll run out of disk space in the root partition?". And here is where LVM comes in handy: you can take some MB/GB from your /home or from your unused partition and add'em to your root.

Also, I would split / (root) from /boot: the reason is that you can use an ext2 for your /boot and ext4 for your / (root). ReiserFS it's wonderful, and it's very fast and reliable with a great number of little files (and that's journaled too). Concerning the NTFS data partition, I would suggest you to mount it in /media/data, so to be easy reacheable from inside Ubuntu.

I hope to see what you will choose in the end: and I wish you best luck for your brave project of dual-booting Ubuntu Karmic and Win7 with such an eclectic partition table!

Regards

After upvoting kotekzot's answer and jet's comment here is the ultimate solution: buy a plain-old front-rack. Sorry, since shopping recommendations are forbidden I won't suggest you a specific brand.

Simply mount the system disk in the rack and swap at necessity. Since these things are cheap, you should consider buying two twin racks, because every model requires you to mount the disk in its unique enclosure that can be extracted at any time. So if you have a single rack and two enclosures you can swap disks at any time. Obviously be aware that if there are any other disks mounted on your desktop the OS has full access (super user principle) to all of them.

To run Windows in a way that it cannot access Linux partitions even after being infected by the worst malware ever, an alternate would be virtualizing, but obviously you won't get direct access to video card and gaming performance.