I am sharing a DropBox Folder with by Friends for a project so that we can be updated with each other progress. But i was not very convince after reading the How secure is Dropbox?. What if one of my friend PC is infected with malware what will be the security impact then. How i can sandbox my shared folder from my rest of my system both for linux and windows. Is it possible or just a myth?
DropBox will transfer files. A file infected with a virus is still a file: a bunch of bytes. DropBox will do nothing special with such files; it will transfer them like any other file. Using DropBox is no different, in that respect, from exchanging the files over USB drives.
Virus for Linux are rare, notably because of the sad state of binary compatibility between Linux brands. The Linux ecosystem has long embraced the source-based publication model, with distribution-specific configurations and patching (the distribution packages). Publishing a binary only software for Linux is challenging, not only for closed-source software vendors, but also for virus writers. Also, a virus requires importing an infected file from the outside and then executing it, something which happens much more often on a desktop system than on a server (because the desktop system has a human user sitting in front of it). Linux is not very widespread as a desktop system (not as much as Windows or MacOS X). These two characteristics, the difficulty of binary distribution and the scarcity of desktop Linux systems, make Linux is a not very tempting target for virus writers, which explains the rarity.
Therefore, to minimize risks of virus infection through your DropBox-obtained files, use Linux. For even more protection, use a more confidential OS like FreeBSD. In particular, if your operating system is distinct from that used by your friends, then risks of infection are very low, because cross-OS virus are extremely rare.
(I am typing these lines on a Linux system which runs on a PowerPC architecture.)
If your friend's PC is infected with malware there is a risk that any file they transfer to you will also be infected. Using an up to date AV solution will minimise this risk, although it won't completely remove the risk as the AV may not find every piece of malware.
It is possible to segregate DropBox, yes - simplest route could be to use a VM and just run DropBox in that.