Unzip HJT into it's own permanent folder before doing anything in order for it to create backups. (Not a temporary folder or directly on the desktop (in a folder on the desktop is fine) & not directly on your hard drive). Close all (browser) windows & rescan with hijackthis. When the scan is finished place a check in the box to the left of the following entries & click 'fix checked' :
R3 - Default URLSearchHook is missing
O2 - BHO: TwaintecObj Class - {000020DD-C72E-4113-AF77-DD56626C6C42} - C:\WINDOWS\twaintec.dll
O2 - BHO: Search404 Class - {53C330D6-A4AB-419B-B45D-FD4411C1FEF4} - C:\Program Files\404Search\404Search.dll (file missing)
O2 - BHO: (no name) - {549B5CA7-4A86-11D7-A4DF-000874180BB3} - (no file)
O2 - BHO: (no name) - {FDD3B846-8D59-4ffb-8758-209B6AD74ACC} - (no file)
O3 - Toolbar: (no name) - {2CDE1A7D-A478-4291-BF31-E1B4C16F92EB} - (no file)
O4 - HKLM\..\Run: [myyeladbtubp] C:\WINDOWS\System32\twugck.exe
Reboot into safe mode following the instructions here & navigate to & delete the following if found:
C:\WINDOWS\System32\twugck.exe-file
Reboot normally after doing the above then post a fresh log please.