Sometime downloading large 100 MB files on my 512 K DSL connection is fast, other times it can take more than an hour. Any suggestions?

There are several reasons why your DSL Internet connection can be slower sometimes.
Your 512 K DSL line is the total download capacity for your household. So, when the connection seems slower than usual, check if someone else in the house is watching a movie on the web, playing an online video game or something else that is competing with your downloads.

If you have a Wi-Fi wireless home network and you did not implement access security, someone else might be stealing some of your network speed. (See, How do I make my home or small business Wi-Fi network safe?)

Make sure you have installed anti-spyware and anti-virus software on your PC and it is working.  Spyware can cause significant performance issues.  (See, How can I protect myself from hackers?)

Also, DSL performance will often vary over time.  One easy thing to do is to unplug your DSL modem power cord for a minute, then plug it in again.  That will force your DSL modem to reset your connection, and newer connections are sometimes faster than if you have stayed connected for a long time.

You can test how fast your Internet connection is at the link below.  It is an online tool from CNET that shows the speed of your connection.  Once you see what your actual speed is, call your Internet service provider, tell them the test results and ask if you are getting the speed you were promised.

For more information, visit the Cnet  website