1. Does your Internet Service Provider (ISP) limit the number of bytes you may download and/or upload during a "fixed" or "sliding" 30 day window? If you exceed this value, your ISP may "throttle" back your access speeds. (I know this is "true" of WildBlue Satellite connections.)
2. You could power reset all of the hardware components in your Internet access path: Modem, Home Router, Home Computer.
a. Gracefully shutdown and power off the computer.
b. Unplug power to router and modem.
c. Wait a minute or so...
d. Reconnect power to modem and wait (as long as it takes - depends on ISP) for it to reinitialize...
e. Then reconnect power to router and wait for it to reinitialize (a minute or so).
f. Restart computer and retest.
If that doesn't resolve the issue, I'd probably let it go a couple of days to see if the problem resolves itself...
You could call your ISP and see if they're aware of any "capacity" issues at their end (but I'm not optimistic you'd get any meaningful answer from them).
3. Perhaps you have multiple security software products "examing" the download stream. Any recent "updates" or changes to your security environment?
What security products are installed?
4. If Windows XP, boot to "Safe Mode with Networking" and check your download speeds. Do not surf the Internet in this mode as most of your security software will be disabled. Just check the download speeds using a "trusted" site.
5. If there another computer in the household you could check your download speeds with?