It's actually easier than you might think. Oneiropsaxtis stated that it can take centuries to break wireless encryption, but that's not necessarily true. If you use so-called Wireless Equivalent Privacy (WEP), your connection is as good as open, because WEP is very badly flawed. The open-source Aircrack-ng suite can break WEP encryption in only a couple of minutes; even WPA (which you really should be using--preferably WPA2) is vulnerable if not set up properly (see the cited source). I tried it myself for a network security class and can attest it does work, at least for WEP. If you read or post your email through non-secure connections (plain SMTP, POP, IMAP; webmail over non-secure HTTP), anyone using a packet sniffer in the vicinity can read the email on your network if it's open or using WEP or WPA with a poorly chosen passphrase. Of course, I realize you're more concerned with the people already on the network, but you should also be concerned with those outside, which the above is meant to address.
For those on the network, there are other ways to capture traffic, even across switches and routers. Some people assume this is not possible (for wired networks at least), because both switches and routers are designed to send traffic only to its destination (as opposed to hubs, which broadcast everything they receive to all ports). But there is a way around this.
You see, Ethernet, Wi-Fi, and similar protocols assign each device a unique hardware address, but these aren't routable. The Internet protocol uses its own addresses that can be routed, as well as the Address Resolution Protocol (ARP) to map between them. The key here is that each device answers for itself, and is assumed to be telling the truth, and that each device also caches mappings in order to reduce traffic and improve speed. An attack called ARP cache poisoning works by sending false mappings so that hosts (e.g. the PCs on your network), switches, and routers send packets to the attacker's computer, instead of the victim's; the attacker can then do whatever (s)he likes, such as read packets, modify them, stop them, or send them on to the correct destination unmodified. This attack works on both wired and wireless networks (though there are ways to prevent it), and software is available that makes it pretty easy to do. This is just one way to get around switching and routing, though if you use a shared key (e.g. WEP, WPA-PSK), those already authorized to use the network can simply sniff the packets from the air and use the key to decrypt them. But of course, if you're using secure connections (e.g. talking to secure web servers), you're still okay.
Anyway, regardless of whether you think the others on your network could or would read your email, you should take proper security measures, such as using proper wireless security (WPA2 if possible, and use a passphrase that can't be guessed except by brute force), don't send *any* sensitive information over non-secure connections, and of course secure your computer against others using it unless you really trust all of them.
Don't forget that even if you can send and receive email through secure connections, it can still be read on the way to its recipient(s), unless the body is encrypted, so don't send sensitive information through regular email. (If you want secure email, you can look into PGP, though a contact must also use PGP or compatible software.)
To address another possibility that was raised: yes, antivirus software may be able to detect *software* keyloggers, but there are also *hardware* loggers available. The obvious downside is that one can be easily discovered (if you're looking for it).
Sorry for being so long-winded, but I thought you could do with some real security knowledge. Not that I'm an expert or anything, but I know more or less what I'm talking about.
Disclaimer: obviously no one here should go and use any tools mentioned on someone else's network without authorization. They are meant for finding security weaknesses so they can be fixed, rather than to enable criminals (they do incidentally, but criminals would have such software anyway, so it's best to have tools designed to help improve security).
A collection of tutorials for Aircrack-ng: http://www.aircrack-ng.org/doku.php?id=tutorial&DokuWiki=b972dd13028964d5934229f2520068e3