flashingcurser, if I understand correctly, original-sin wants to do the opposite.
O-S wants to connect to Internet THROUGH WINDOZE! ARE YOU NUTS?
Anyway, yes you can do that. On Windoze, you need to enable Internet sharing; look up google if you don't know how to.
On Slackware, you need to set your Windoze machine IP as your gateway. And set your ISP DNS as your DNS. That's all. Oh, make sure both the machine is on the same subnet. I should think you know that already.
PS:
By the way, Windoze XP doesn't do a good job in ip forwarding (NAT, internet sharing, etc; whatever you call it). It is SLOW. Well for a dial-up connection you probably won't notice the difference.
I tried using Windoze XP on P4 2GHz machine to share a 1Mbps ADSL comparing it with Power Mac G3 266MHz running Mac Os X.2.8. Guess which machine give me more response and speed? If you guess the punny PowerPC G3 266MHz machine; then you got it right.
The lesson of the story is, if you can fix the modem to your Slackware, then do it. Share Internet from Slackware to Windoze NOT vice-versa.