If you disable AVAST, are you able to login? Also, are you able to login to the Demo database? It would be called ACT(version#)Demo and by default is located at C:\Documents and Settings\All Users\ACT\Act for Windows (version#)\Databases. The user name is "Chris Huffman" and there is no password. If you are able to login to the Demo, then the issue is with the username and/or the password for your database.
Note: Effective 6/1/13, Sage no longers provides support for the Act! software. This is now provided by Swiftpage.