test your computers ability to download a small test file!!
if you're having troubles with disabling your computers DAP or firewall, test it by downloading
*** this small test file. ***
if it works, you can download the videos without a problem...

download accelerator problems i.e. 5.0 - 6.x:
if you are not able to download a video, and after clicking
the download link you are sent to another page, you have a
download accelerator running in your browser. below are a few articles on how to resolve this issue:

from: http://www.speedbit.com

How can I uninstall Download Accelerator Plus?
To uninstall Download Accelerator Plus, either use the Uninstall utility under DAP's program group in the Start menu, or the Windows Control Panel "Add/Remove Programs" utility. The change will take effect after you restart your browser. NOTE: Please make sure to exit Download Accelerator Plus before starting the uninstall operation.

How can I temporarily disable Download Accelerator Plus?
Press the ALT key (while clicking on a hyperlink) in order to temporarily disable the browser integration of Download Accelerator Plus.

from miscrosoft.com

Error Message When You Try to Download a File: Download Accelerator Error

SYMPTOMS
When you start Microsoft Internet Explorer and attempt to download a file over the Internet, you may experience the following behavior:
* An additional toolbar is displayed in your browser window, and the File Download dialog box looks different than the Internet Explorer File Download dialog box.
-and-
* When you click Start Download in the Download File Info dialog box that is displayed on your screen, you may receive the following error message:
Retry the download of file name
When you click Retry Now , you may receive the message again. After you click Retry Now several times, you may then receive the following error message:
Download Accelerator Error
Connection to site failed: An invalid argument was supplied while trying to download file name
where file name is the name of the file that you are attempting to download.
As a result, you may be unable to download the file. CAUSE
This behavior can occur if Download Accelerator Plus from SpeedBit is installed on your computer.

RESOLUTION
To work around this issue, uninstall Download Accelerator Plus from your computer. To do this, follow these steps:
1. Click Start , point to Settings , and then click Control Panel .
2. Double-click Add/Remove Programs .
NOTE : Because there are several versions of Windows, the following steps may be different on your computer. If they are, please consult your product documentation to complete these steps.
3. In the Add/Remove Programs dialog box, click Download Accelerator Plus in the Currently installed programs list, and then click Change/Remove .
4. Click OK in response to the following message:
This uninstall must reboot your computer to replace/delete files that were in use. Press the OK button to reboot your computer.

... also
FIREWALL issues: if you experience the problem of receiving only a small file, or being sent to the sign up page after clicking on the link to download the movie, you may be behind a firewall. you will have to temporarily disable the firewall to download the videos.