Password-protecting the folder won't really do a whole lot if they can just log in as an administrator and claim ownership of the directory themselves.
I'd suggest installing TrueCrypt, and using that to create a file-based encrypted volume. Then put your DropBox directory inside that, and only folks who know the decryption password can get to it.
Note that they could easily install a keylogger on your PC to learn your decryption password without your knowledge.
Also, the requisite disclaimer: Check with your IT department, and don't do anything contrary to established IT policy. Oftentimes organizations frown on personal use of work property (your PC) and resources (the bandwidth used to download from/upload to DropBox), so make sure you aren't violating any rules that could land you in trouble.