AIM is considerably close to the iChat experience for Windows, and contains many of the same features as you get with iChat.
If you are looking for an iChat alternative that offers access to multiple IM services, I would also recommend the following multi-protocol IMs as your best iChat for Windows alternatives:
- Trillian Astra: My most favorite of the multi-protocol IMs. Trillian Astra is fun, easy design and customization features. I would try Trillian Astra first when looking for a iChat for Windows alternative.
- Digsby: Extremely versatile and fun to use, Digsby also offers users the ability to check email, update your Twitter account and even access RSS feeds.
- Pidgin IM: One of the oldest multi-protocol IMs is also one of the most fun. Pidgin (formerly GAIM) offers a light, clean IM experience with access to all the major market IMs, plus MySpaceIM.
- Miranda IM: Another light and simple IM, Miranda IM is barebones but allows users to chat with many friends across IM networks without taking up loads of hard drive space.