I do not agree with Vista being an issue, I have 3.0/1.5 Mbit and I am downloading up to 3.5 on a good day, 3.3 on a normal day and my machine is vista.
I am also at the furthest point on my loop, you may want to check a few things first.
(1) Your filter is on the phone and NOT on your DSL modem.
(2) Check your phone line for noise, pick up the phone, dial 1 and listen for crackles and weird noises, report to phone company if so.
(3) Plug your modem direct into the box on the outside of the house, check your results.
(4) Check for Virus and spyware on your computer.
(5) The last thing, check your line loop length, your DSL/PHONE company should be able to inform you by doing a line test on their end.
Also, who is the DSL company, do you know your loop length? how many phones plugged into your phone line? you should have no more then REN (Ringing Equivalence Number) of 4, 4 phones with a REN of 1, 3 phones and an answer phone could = 4, noise from lighting, motors, microwaves and so on could affect your signal.
If they are stating you should get 1Mbit, then call them and have someone come out and do more tests, if they say you should get it, then by all accounts you should be able to.
Good luck, keep us informed.