I know of only one way that this can happen in windows, and has nothing to do with Steam other than their not-so-great decision to install all content into Program Files.
If you are on Windows 7 or Windows Vista, and you have UAC set to ON, then change it later to OFF, this can happen.
UAC does thing evil thing where if a program tries to write to a "protected" area like Program Files, EVEN IF YOU ARE ADMINISTRATOR, when you get a UAC prompt and give permission, it actually writes any files that would normally go into that directory into a "virtualized" directory somewhere else.
If you turn off UAC, that will no longer happen, and as an awesome bonus, it will no longer know about the files that it virtualized.
So in your case, if you had UAC on, installed a ton of games, then later got sick of UAC and turned it off, this would happen as you described.
See the second paragraph in "Features" in the article on User Account Control.
If you have done this and turn on UAC again, your content will come back, but be a TOTAL mess because if you've downloaded more stuff with UAC off, then that will be invisible when you turn UAC on, and vice versa.
this drove me a little crazy once before I realized what was happening.
i'm curious to know if this is your issue.
This is happening because the partial-game is being downloaded into cache, and upon completion, its moved onto your system.
When you restart your PC, cache and temp files are cleared, thus losing the games download progress.
EDIT: Im not sure if there is a workaround, if there is, I wouldnt be familiar with it.
EDIT2 Try this
Load Steam
From the Library section,
right-click on the game and select Properties from the menu.
Select the Local files tab and click the Verify integrity of game cache... button.
Best Answer
In
Preferences->Downloadsthere's a number of options which could cause your problem. The two most obvious are...Try a large download from another site outside your school's network, perhaps another game utility. For example, does Origin or Glyph have the same problem? If they do this would indicate it's not Steam but something about your computer or network.
You should also check your total throughput of your connection using a site like speedtest.net. Be careful to note the difference between Mbit/s (megabits per second) and Mbyte/s (megabytes per second). Speedtest, and most ISPs, will report in megabits. Everyone else uses megabytes. 8 Mbits == 1 Mbyte.
Check if Steam is being blocked or throttled by a firewall or anti-virus, either on your computer or on your router. Conversely the problem could be a virus; if you don't have an anti-virus, get one.
If you're on wifi, try a wired connection. This will avoid interference from various sources.
Finally, the behavior you mention where it starts fast and then slows down is common among some shady ISPs (and some big ISPs are shady). This lets small downloads happen fast, but large file downloads get throttled. Web pages are made up of lots of small files, so they load fast. This lets most users have a responsive experience, but penalizes people downloading large files (and thus using a lot of bandwidth).