If you have ever used RunAs utility then SafeLauncher is the best choice for you. It lets you organize your favorite applications and provide a fast and simple way to run them under Windows account you specify. Futhermore it may greatly help you to increase the protection level of your computer. You may run any program (Internet Explorer, ICQ, etc) under the guest account while still working under the account with administrator's privileges.

If you would like to give your .NET application a simple way to communicate with remote computers then BytesRoad.NetSuit may be the library you are looking for. It is an open source network library developed for the .NET platform and is completely written in C#. It exposes classes that you may use in your own application for communication with FTP servers. It also contains classes suitable for TCP/IP communication through various proxies.