Either your wireless modem is unstable, or your adapter is.
Some questions.
1. Is any electronics products near your computer or wireless
modem? Like handphone, TV, etc.
2. What brand and model of wireless modem and adapter are you
using?
3. Is there any specific patterns that this occurs? Like in the
morning or night, or no specific patterns?
4. When MSN disconnects, are you able to surf the Net?