any number of things could affect your phone lines, from bad wiring, line noise, modem problems, isp problems, etc.
 
try this:
 
when you open your dial-up dialog where you click "connect", instead click "properties", located the drop down menu where you modem is selected, click the "configure" button and check the following:
 
-Enable Hardware Flow Control/LCP extensions
-Enable modem error correction
-Enable modem compression
 
try doing that and if it don't help come back and i'll give you a few more tips.

What speed(s) are you connecting at and what's the max supported by your modem ? Meanwhile, after checking EddiE's suggestions you can try the following too:
 
- Your modem is correctly installed, no exclamation marks in device manager, right ?
 
- Test other ISP's, line quality varies.
 
- Bad pings (if you mean you get replies but they are slow)are propably not caused by a bad phone line, but by bad or crowded ISP. What are your pings ? Get one of the visual ping tracing tools (try Tucows and the gang) and see where on the route your problem starts.
 
- Get a download manager (Go!zilla, Flashget etc), they open several download channels at once and you can utilize your bandwidth to max
 
- have the phone company check the quality and the "noise" on the line. Check the wiring & cables (especially self made ones)
 
Wouldn't ISDN/ADS/Cable be an option ??
 
H.