Help removing .kext files

I am running 10.7.5 on an old MacBook and lately the fan has been running a lot. Cutting to the chase, I've done all the necessary system exploration and learned that I am still running some kernel extensions for applications that no longer exist on this machine, namely Sophos and VirtualBox. When I type in Terminal:

kextstat | grep -v com.apple

I get this information:

50 0 0xffffff7f809ca000 0x2a000 0x2a000 com.seagate.driver.PowSecDriverCore (5.1.1) <49 48 47 32 12 4 3>
95 0 0xffffff7f807b3000 0x7000 0x7000 com.sophos.nke.swi (9.0.3) <4 3 1>
113 0 0xffffff7f807ba000 0x3000 0x3000 com.bresink.driver.BRESINKx86Monitoring (8.0) <5 4 3>
114 3 0xffffff7f82763000 0x46000 0x46000 org.virtualbox.kext.VBoxDrv (4.3.10) <7 5 4 3 1>
115 0 0xffffff7f81a38000 0x8000 0x8000 org.virtualbox.kext.VBoxUSB (4.3.10) <114 35 32 7 5 4 3 1>
116 0 0xffffff7f8184a000 0x5000 0x5000 org.virtualbox.kext.VBoxNetFlt (4.3.10) <114 7 5 4 3 1>
117 0 0xffffff7f81a40000 0x6000 0x6000 org.virtualbox.kext.VBoxNetAdp (4.3.10) <114 5 4 1>

So then I try to remove a certain file, like so:

sudo rm -rf org.virtualbox.kext

and it asks for my password and the procedure appears to run smoothly. But after restarting the machine and checking again to see what .kext files are in terminal, there it is again. So my effort to remove it was totally unsuccessful. There are no VirtualBox or Sophos folders left in my system, so there is no uninstaller to run. How do I get rid of these files from my system once and for all?