HP Presario 2100US Notebook Specifications
Integrated 56 KB modem and Integrated 10/100BASE-T Ethernet LAN (RJ-45 connector) are mentioned but nothing about WiFi or 802.11 (WiFi spec).
Yeah, based on what's listed in the specs the laptop does not come with a wireless adapter. You need to install one in the PC card slot. Back then wireless was not standard.
You can probably use a USB WiFi Dongle like this one
It's a USB 2.0 device but should be backward compatible with your USB slot.
There are also PC Card Type II/III Wifi adapters that you could use.