Hm...
I would strongly advise running weekly 'security checks' to be safe. AVG and Ad-aware are both free programmes I would reccommend, as well as whatever other safety checks you have running.
It probably would also be wise to do a fortnightly 'full sweep' of your computer - defragment, recompiling, virus, adware, spyware... make sure it's clean. You can also prolong your unit's life by keeping it clean and away from sources of dust.