Bros,

I am presently using a Macbook pro, but am wondering whether my skype and msn conversations could be viewed by other people in my office ?

Company running on a wifi network. Any ideas?

tapping is possible but most probably they can only get the encrypted messages

MSN messages are sent in plain text and easily logged/viewed. Skype should be fully encrypted. Wifi networks can be sniffed passively by anybody with the WEP/WPA key.

wifi hack is possible..

Any advice on how we could avoid these? I don't think a antivirus would be sufficient.

use VPN is the answer.


How VPNs Work
http://computer.howstuffworks.com/vpn.htm

Regardless of what OS you use, if you're transmitting packets in clear text without encryption, it can and will be tapped.
Major companies in Malaysia, banks, telco's etc these days have deployed network equipment, like BlueCoat which works like a vacuum cleaners, sucks and retains everything, including VOIP packets. It's able to replay the packets, construct and analyze.

If you do need to chat, use Skype...and you need to call use Skype, even send or receive files. Up to this point, Skype packets are encrypted using proprietary encryption keys. Hence, unless you work for NSA, it's almost uncrackable.

You're running on WiFi, make sure that it's on WPA2, the very least WEP. A simple idiot tool like FireSheep could sniff your session, from an unencrypted session. VPN is also another option, paid versions are the best, reliable and fast - BolehVPN or HideMyAss.

Once you run a VPN, it 'blinds' network equipment like BlueCoat, but if you have a strict network admin, he might go and look for the person who is connecting to a VPN.

Use SecurityKISS VPN. It is free.
More info, here.

The free version allows traffic of 300MB (Download+upload) per day.