Of course it will go down if you are not upgraded to drivers/programs for your modem to use broadband properly or XP. You should have noted the I/D numbers on the modem card & who made it. You then go direct to the manufacturers web site & download the proper drivers/programs for your particular modem.
Try click here or click here both of these are tech sites usually free but you will need the make & I/D of your modem card.
Failing that purchase a new modem card from your local shop with installation floppy or CD & start properly.
bear in mind that microsoft do have compatability protection built into XP so you have to upgrade all drivers for all parts in your PC to take full advantage of broadband & your PC