Run the ltremove.exe file you have in the unzipped driver package. This will remove the old drivers which you must do before trying to install new ones.
Then download these drivers:
http://www.agere.com/support/client/modem_driver2.html
Copy the file to a floppy, take it to the computer, copy it onto the hard drive, and run it. It's a self-installer, not .inf files to go with the add hardware wizard as the Aztech drivers are.
These are the latest generic Agere (Lucent) drivers for all V.90 Lucent/LT Winmodems. That's one reason the only Winmodems I will deal with without complaining are Lucents - universal driver works fine.
NOTE that WinXP should have native Lucent winmodem drivers - and updates are available through Windows Update. Although the Agere page does not mention that this driver I'm pointing you to supports XP, I get the impression from Richard's Modemsite that it will in fact work.