How did you install XP was it an upgrade??
If it yes you should have done a clean install are you running Fat32 or NTFS?? my older computer
run Windows ME so when i put XP in i kept my ram at 320meg as for modem you do not say if it is dial up or broadband if it dial up could be your driver i have a Zoom modem but it works better on XP with Acer driver.