PC speed issue Help!

I have a couple of PCs and a couple of tablets and wireless printers on my WIFI network.  Everything is working great,  except for my newest PC.  Im running windows 7 and just installed a new Sabrent high speed wireless adapter because of very slow download speeds on this machine.  I have Xfinity internet and get speeds of 30 to 50 MBPS on my tablets, phones and the older Vista OS PC.  I have a strong signal but the speeds on the windows 7 machine run anywhere from 1 to 20 MBPS.  I have scanned for malware and found nothing.  I even bought one of those PC speed booster programs which indicated something like 1500 issues which were identified and "fixed".  The "fix" did not fix the problem.  I know my network is working well but I can't get my wife's PC to download any faster than a snail's pace.  I generally use Chrome but the problems is the same with IE.  Suggestions???

As far as I can ever remember, there is no such thing as a pc speed booster.  That program whatever it is is likely crap and slowing down your pc, while making money for someone else.  If your tablet and other pc's are working fine, so should this computer.  You may not have found the malware, but I'm guessing it is there.  A new adapter shouldn't be needed either.

The best possible solution would be to reset to the original factory defaults if you can.  Usually a function key at startup, or a bootable CD or something like that.  Set it back to defaults, and if it works jsut fine, then it's something that got added to it that slows it down.  Remove unnecessary software, never use McAfee anything, and dont install freeware programs that are untrusted.  This one is especially hard, as they are very good at deception. 

Try one more program if you will, and see if it helps.  It removed something that I couldn't find/remove at all with my usual suite of programs:  Called ADWCleaner, and this is the official website:  http://www.bleepingcomputer.com/download/adwcleaner/

Martin,  have you tried the xfinity speed test site yet?  http://speedtest.comcast.net/  .  Test it there first and then if you are still getting mixed results you should call Comcast support and they might be able to pinpoint where the problem could be.  It could be just a configuration issue.  Al is right about that pc booster app.  You don't need it.

Thanks Al.  No luck so far. Norton wouldn't allow ADWCleaner to install. I cant seam to find a boot disk and I can't get the setup screen to come up whne I hir the del key during boot up.  I did have some malware called Rocket something but I removed it.

I knew I didnt need the booster but after hours of unproductive solution searching I gave in.  Still not working right. COMCAST won't  help.  The network is workjng great.  Its an issue with 1 PC.

Are you connecting using cable? CAT5E or CAT6? Check them I had same issue and found cable was munch. If not try to just erase everything and reformat your computer.

No cat 5 or 6.  All Wifi.  Speeds are really fast on all other devices.

Remove Norton, Remove the speed booster, uninstall the crap out of everything you can on it. Then run that program I mentioned.   I know that some programs will detect it as a problem itselt, but I swear, it's not and can fix a lot of issues.  What is the manufacturer of the PC, and Model number?  Any new computer should have an easy way to reset to factory defaults, should be an easy google search too.  It's really funny thinking Comcast will help you! 

That case, I agree with Al, restore computer from manufacturers defaults, if you are using Windows 7 it should have the restore in your hard drive. This will eliminate any malware or incompatible programs.