Have you or are you using a third party download manager, or do you experience this in internet explorer? What version? Try running IE without add-ons
(usually rt. click on icon) and see if you can download. If so, there is an issue with one of the add-ons. See Don Varnau's post in this discussion.
Maybe reset IE to default settings as described here.
Then check downloading.
Do you have Spybot running the resident SD helper or tea timer? If so, try disabling them and try another download. If you've got the spybot BHO running as an addon, disable it. Check downloads.
Have you cleaned your temp files and temporary internet files out recently? Go to your browser, open tools>internet options>general tab>settings (browsing history) and make sure you have at least 50 Mb storage. Clear the cache. (Your cookies will be replaced). Run disk cleanup or ATF cleaner to do so if you wish..
Take a look in the Event Viewer and see if there are errors reported there in connection with the download. Use the error id code to find solutions for this.
Otherwise, I'd guess some security program is interfering with the writing of the files. Usually at 99% the file has been downloaded to a temp file, and is being written to the location you chose to save it. It is possible you have checked a setting in your AV or firewall that is preventing files from being written to your folders. Perhaps McAffee is scanning the download. How long have you waited? Set all to default and try again.) (See here)
Avira makes a good free AV. Eset NOD32 is also very reliable, and lighter on system resource use. See AV comparatives.
If you have a Belkin router, there have been reports of problems with the .30 firmware causing this error. See here.
You may have to roll back to .29.
Have you applied the latest service pack (sp3) and updated the system at windows update? Always worth a try.
If none of that helps, I'd suggest you follow the steps in this article
to optimize your system, and possibly run sfc /scannow to check the integrity of the system files.
Ok that's enough. Good luck.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
If I've managed to answer your question or solve a problem, please take just a moment to rate this post....thanks!