Did it come preinstalled and did you use it in the past? If not, have you been using another anti-virus? Some manufacturers like Dell, HP, Gateway include anti-virus protection preinstalled on the systems they sell.
If it came preinstalled, it may be on the disk you received which includes other software. You can also directly download the software from Trend Micro's site or from here
If you didn't get a CD then it may be part of your Recovery Disk
or included in a Recovery Partition