Heart of Winter is the expansion set to Icewind Dale. You can't play it without Icewind Dale. Look for it at your local computer games retailer, or buy it from
the Interplay site.
There's another expansion called Trials of the Luremaster, which is available for free over the internet as a "free" download (it's not really free, you'll be paying your ISP in download time). You can download that at
www.bioware.com.