I noticed that the internet download speed  suddenly slowed down last night. It was around 80 to 90 kbps before, it changed to around 20 kbps. The internet speed is still roughly the same though. When I tried a speed test, the download speed REALLY reduced but the upload speed remained kinda the same. What could the problem be here? 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?What ISP do you have and what type of connection is it?
Many ISPs these days are throttling download speeds during heavy traffic hours.Don't you have some adware / malware eating your connection?
Try netstat -a to see all connections and check if they are ok.Or it could have just been a problem with the server from which you were downloading. Or was this a torrent?

It may not have been your fault at all.