One of the causes of the slowdown, especially at startup, can be Windows trying to restore connection with network share. Configure Registry to prevent it:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\NetworkProvider] "RestoreConnection"=dword:00000000

If there's a performance problem, disable all the unneeded services and startups, use a lightweight browser (see memory usage), disable Aero theme, etc.

If you suspect malwares have infected your PC, it is advisable to format you're drive (and not trying to clean it).

Also, USB pendrives can prevent the system to boot. Unplug them.

One of the causes of the slowdown, especially at startup, can be Windows trying to restore connection with network share. Configure Registry to prevent it:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\NetworkProvider]
"RestoreConnection"=dword:00000000

If there's a performance problem, disable all the unneeded services and startups, use a lightweight browser (see memory usage), disable Aero theme, etc.

If you suspect malwares have infected your PC, it is advisable to format you're drive (and not trying to clean it).

Also, USB pendrives can prevent the system to boot. Unplug them.