Win7 64bit Large file download slowing system (NOT bittorret)

hiya guys. I have a friend that usually comes to me regarding tech issues and I'm a bit lost on this one. He has a Windows 7 64bit OS with 4GB of ram. He has been downloading a set of 8GB files and almost like clockwork, his entire system starts chugging at about 3GB. He is NOT using a bitttorent. He is using a download accelerator and he has tried a different accelerator to eliminate the possibility that it was the software. He does have comcast, but he says that in the past, he's never had issues with traffic shaping. Also, his SYSTEM is slowing down (anything else he does gets really choppy) not just the download, so we think that indicates it's not just the normal ISP stupidity, it's something wrong on his system side. What do you think guys? Any suggestions on things to check out?

Just out of curiosity are the files .MKV files?
Does Explorer.exe go up to about 50% and start chewing up his RAM when he downloads these files?

well, it's not a bittorent so he's not using a bittorrent client. He's turned off his antivirus. He's scanned his system with adaware so there shouldn't be any malware. A little more background. His cpu and ram are not listed as maxed out. He was using freedownloadmanager as the downloader. He tried to download the file in internet explorer without a manager. At 3gb his system spiked and went a little crazy, then leveled off and continued at the correct 15mbps speed. So there is an issue there, but IExplorer seems to have compensated somehow but the accelerators can't seem to deal with it. Hope that more info helps. Of course he could always just use IExplorer but it would be nice to track down the culprit so he can have the safety of a resumable accelerator