From Google Health I read in Yahoo HealthDay that:-
"Along with its other benefits, an aspirin a day may help keep head and neck cancers away, a new long-term study suggests. It wasn't so much the amount of aspirin that was taken but the length of time that people were on the drug that mattered. Taking aspirin for under 10 years did not have a significant effect but taking it 10 years or more was associated with a 30 percent reduction in cancer" said study co-author Mary E. Reid, an assistant professor of oncology at Roswell Park Cancer Institute, in Buffalo, N.Y.
But it's still too early to recommend routine use of aspirin as a cancer-preventive measure, said Dr. Michael Thun, vice president for epidemiology and prevalence research at the American Cancer Institute.
Aspirin's cancer-fighting effect appears to be linked to the molecular mechanism by which NSAIDs fight pain. The medications inhibit an enzyme called cox-2, whose activity is known to increase in pre-malignant lesions. Inhibiting the enzyme may also help slow the proliferation of cells.
Anyone who wants to take aspirin as a cancer preventive should first consult their physician, Reid cautioned. "You should always talk to a physician before taking anything chronically," she said.
Taking aspirin each day does have its risks, Thun said. "At this point, aspirin is still not recommended for prevention of any cancer because of the risks of bleeding, particularly gastrointestinal," he said. "What is still missing is proof that the benefits of taking aspirin outweigh the risks."
More info on Aspirin & Cancer: National Cancer Institute