You've most likely got a malware infestation of some kind, though you may not actually have a virus. Some of these popups are generated by malicious software trying to get your credit card details. If it's coming from an antivirus package that you Know you have installed in your computer, It's probably legitimate. However if the alert keeps coming up, it's best to get a professional to look at it regardless.
This varies from year to year, and some packages can get worse while others can get better. Having said that, Avoid Norton. For more info, go to AV Comparatives, an independent antivirus performance website. For a current recommendation of an antivirus package, contact me.
Yes and no. Sometimes the comparitive increasing speed of technology makes us think our existing machines are getting slower when in fact everything else is speeding up. On the other hand, most computers develop problems and accumulate junk over time which slows them down, particularly if you install a lot of different programs on them. General system optimisation is recommended.
Though this is best left to an expert, what you can do is take a look at this guide: Windows Cleanup/Speedup Guide.
Most of the time this is hardware related, which may or may not be easy to fix - a hands-on diagnosis is warranted in either case. Sometimes these can be software problems or easy-to-fix hardware issues.