Hiii

It is better to ask only one question per thread.

system.gc.collector can be used to call an garbage collector

When the instance of a class is no longer in use the Garbage Collector is explicitely called.
It is called so that garbage collector will release the resources which are not in use.

To load assembly at run time ,follow the below links.
http://support.microsoft.com/kb/837908
http://www.codeproject.com/useritems/Assemblies___Appdomains.asp
http://www.kdkeys.net/forums/1739/ShowThread.aspx