Slow System

Operating System: Windows 7 Home Premium 64bit SP1

Hard Drive Partitions:
   1) C: (96.15 GB) - Healthy (Boot, Page File, Active, Crash Dump, Primary Partition) - this is the partition on which I have installed the system and marked as active.
   2) D: (200.84 GB) - Healthy (Primary Partition)
   3) System Reserved (F:) (1.00 GB) - Healthy (System, Primary Partition)
   And 101 MB Unallocated; this partition is not shown in the Disk Management Panel. 

Symptoms:
   I had re-installed the system with the Bootable CD. It had been fast until it was infected with a virus caused by a software. I have scanned my system with Malwarebytes, Spybot, ComboFix, AdwCleaner, HijackThis, OLT, et cetera and cleaned all the detected objects, but the system is still slow.
   I have also installed all the required updates and updated all the drivers.
   It takes a long time to open an application; and sometimes it is not responding. When I open Microsoft Internet Explorer, the IE Window pops up but is not responding and closes automatically. 
   I can use only another Internet browser, but it is also very slow. When I open a web page in this browser, the web page is fast at the beginning but will stop for a long time when I drag down the page and other tabs in the same window will become blank. It will become normal again after about 5 to 10 minutes.
   The system can starts up and shuts down normally. 
   There is one thing that I have noticed: It is shown under the Processes of Windows Task Manager two processes: csrss.exe (Memory - 6276K) and winlogon.exe (Memory - 1412 K), which show no user name and no description. Their CPU usage is usually below 10. I have used Process Explorer to trace their location, but to no avail. I can not terminate or do anything to them either.
   I have scanned the whole system including all files with numerous antivirus, antispy, antitrojan software but they did not detect any threats; but my system is still very slow.

Is my system still infected with the virus? What is the cause of the issue? How can I fix this issue?

Thank you in advance for any help.