There can be several reasons why your download speed is low.These can include, amongst other things, poor network settings(still tweaked for dial-up) and incorrect duplex settings. If you are using p2p software your upload may be getting maxed out which really slows the downloads. Robin Walker has an excellent site which can help you with many problems.
http://homepage.ntlworld.com/robin.d...ips/index.html
The speed you are getting is spot on for a 300k connection. You may be getting this instead of the 750k speed you thought you was on. Have you done the speed tests suggested earlier by Paul?