[iBook] My email address is been used by somebody else

[Sean Finnegan]

My email address is been used by somebody else. They are sending out  
emails to individuals and undisclosed Recipients. How is this  
happening, is it a virus or a trojan, what can I do ?

All help and advice appreciated

Version 10.4.11
  Machine Name:	iBook G4
   Machine Model:	PowerBook6,5
   CPU Type:	PowerPC G4  (1.2)
   Number Of CPUs:	1
   CPU Speed:	1.33 GHz
   L2 Cache (per CPU):	512 KB
   Memory:	1.25 GB
   Bus Speed:	133 MHz
   Boot ROM Version:	4.8.7f1
   Serial Number:	4H51907SS85