ASC-US Pap test results are common and can be related to a host of things; HPV-related cell changes are one possibility, but others include those that might obscure a cell sample: the presence of yeast or menstrual fluid, recent intercourse, tampon use, or douching, or a mistake in the prep of the sample.
To sort this out, it's reasonable to have a woman return later for another Pap (usually in 4-6 months), or to do an HPV DNA test. The way it normally works is if the woman either has another abnormal Pap test (ASC-US or higher) or is positive for "high risk" HPV on the HPV test, she's then referred to colposcopy. Sometimes healthcare providers don't stick to this to the letter, as you have to take individual factors in common, so ask your provider to navigate you through what's right for you.
It's uncommon for "high risk" HPV to progress to cancer to begin with, and very rare indeed for an ASC-US Pap to turn out to involve invasive cancer. Cancer is a rare outcome of "high risk" HPV infection.
I hope this helps some. Read the posts on our HPV board and follow up with more questions if you like.