Need help, Trojan, *urgent*

Alright, here's the problem I'm having... I don't remember exactly what I downloaded, but something gave me this (It was detected by Microsoft Security Essentials) -
TrojanDownloader:Win32/Renos.JS
I said "meh, whatever" and clicked remove. It said successful, but about an hour later, the same thing was back again. I kept clicking remove, it kept saying it worked, and it kept coming back. Now Microsoft Security Essentials is saying "Microsoft Security Essentials isn't monitering your computer because the program's service is stopped. You should restart it now." Sure. So I click the big red button which says "Start" and I get this..
Couldn't start the Microsoft Security Essentials service - Access is denied. Error code: 0x80070005

Now, every time I boot my computer up and login to a user, explorer.exe no longer runs on startup, so I am forced to manually start it. On top of that, my default browser (Mozilla Firefox) crashes every time I open it. Did I mention my internet connection isn't reliable at the moment, and could disconnect at any time? Headache after headache.. Please, PLEASE, someone help me fix this crap. It's really getting on my nerves. Thanks..

~TA

Manual removal is not recommended for this threat. To detect and remove this threat and other malicious software that may have been installed, run a full-system scan with an up-to-date antivirus product such as the Microsoft online scanner (http://safety.live.com). For more information, see http://www.microsoft.com/protect/computer/viruses/vista.mspx.

System Changes

The following system changes may indicate the presence of this malware:

• The presence of the following registry modifications (or similar):
  Value: MSFox
  With data: <full pathname of Win32/Renos<variant>>
  In subkey: HKCU\Software\Microsoft\Windows\CurrentVersion\Run
  
  Value: Str<digit>
  With data: <base64 encoded string> (for example, "x6tveq8ngbtmpknqirnnqauudxwx")
  In subkey: HKLM\Software\Mozilla\MSFox

I'm 99.9% positive it isn't malware. I've not yet seen malware that can control the Microsoft Security Essentials GUI..

Manual removal is not recommended for this threat. To detect and remove this threat and other malicious software that may have been installed, run a full-system scan with an up-to-date antivirus product such as the Microsoft online scanner (http://safety.live.com). For more information, see http://www.microsoft.com/protect/computer/viruses/vista.mspx.

System Changes

The following system changes may indicate the presence of this malware:

• The presence of the following registry modifications (or similar):
  

  Value: MSFox
  With data: <full pathname of Win32/Renos<variant>>
  In subkey: HKCU\Software\Microsoft\Windows\CurrentVersion\Run

  
  

  Value: Str<digit>
  With data: <base64 encoded string> (for example, "x6tveq8ngbtmpknqirnnqauudxwx")
  In subkey: HKLM\Software\Mozilla\MSFox

TheAnonymity said:

Alright, here's the problem I'm having... I don't remember exactly what I downloaded, but something gave me this (It was detected by Microsoft Security Essentials) -
TrojanDownloader:Win32/Renos.JS
I said "meh, whatever" and clicked remove. It said successful, but about an hour later, the same thing was back again. I kept clicking remove, it kept saying it worked, and it kept coming back. Now Microsoft Security Essentials is saying "Microsoft Security Essentials isn't monitering your computer because the program's service is stopped. You should restart it now." Sure. So I click the big red button which says "Start" and I get this..
Couldn't start the Microsoft Security Essentials service - Access is denied. Error code: 0x80070005

Now, every time I boot my computer up and login to a user, explorer.exe no longer runs on startup, so I am forced to manually start it. On top of that, my default browser (Mozilla Firefox) crashes every time I open it. Did I mention my internet connection isn't reliable at the moment, and could disconnect at any time? Headache after headache.. Please, PLEASE, someone help me fix this crap. It's really getting on my nerves. Thanks..

~TA

Microsoft recommends either a-squared Free or mailwarebytes..

Both Free Programs. I had the same bug and this took care of it.

Jacee is the resident subject matter expert on this topic. You are well-advised. There is simply no one else on this forum with more expertise in the subject.

Fisrtly apologies for my ignorance as i am new to all things pc. I also have the same issue whereby it wont remove - it seems like it has been removed but when opening internet explorer and looking in the history random sites appear that have never been visited - i already have a norton product installed on my machine and the trojan was picked up by windows defender - although still appears even after removal - what are the quickets and easiest steps for a novice like me to resolve the issue - any step by step process would be greatly appreciated

sorry in addition i did run a full system scan with the norton product but it came back with no results