My computer is constantly running at 100% CPU usage. Can anyone please help?

Logfile of HijackThis v1.98.2
Scan saved at 2:48:23 PM, on 2/11/2004
Platform: Windows 2000 SP4 (WinNT 5.00.2195)
MSIE: Internet Explorer v6.00 SP1 (6.00.2800.1106)

Running processes:
C:\WINNT\System32\smss.exe
C:\WINNT\SYSTEM32\winlogon.exe
C:\WINNT\system32\services.exe
C:\WINNT\system32\lsass.exe
D:\Sygate\smc.exe
C:\WINNT\system32\svchost.exe
C:\WINNT\system32\spoolsv.exe
D:\Avast\aswUpdSv.exe
D:\Avast\ashServ.exe
C:\Program Files\Executive Software\Diskeeper\DkService.exe
C:\Program Files\Common Files\EPSON\EBAPI\SAgent2.exe
C:\WINNT\system32\svchost.exe
D:\GoBack\GBPoll.exe
C:\WINNT\system32\nvsvc32.exe
C:\WINNT\system32\regsvc.exe
C:\WINNT\system32\MSTask.exe
C:\WINNT\System32\WBEM\WinMgmt.exe
C:\WINNT\system32\svchost.exe
C:\WINNT\Explorer.EXE
D:\Avast\ashDisp.exe
D:\Avast\ashmaisv.exe
C:\Program Files\iolo\System Mechanic 5 Professional\StartupGuard.exe
D:\GoBack\GBTray.exe
C:\WINNT\system32\taskmgr.exe
D:\Opera\opera.exe
C:\Documents and Settings\Tom1\Desktop\SP4Express_EN.exe
d:\c122e15e84adbd8f283df\update\update.exe
D:\HiJack This\HijackThis.exe

R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page =
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,SearchAssistant =
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Local Page =
R0 - HKLM\Software\Microsoft\Internet Explorer\Main,Local Page =
R3 - Default URLSearchHook is missing
O2 - BHO: Google Toolbar Helper - {AA58ED58-01DD-4d91-8333-CF10577473F7} - c:\program files\google\googletoolbar1.dll
O2 - BHO: IEPlugin Class - {CF7C3CF0-4B15-11D1-ABED-709549C10000} - D:\Advanced System Optimizer\Advanced System Optimizer\IEHelper.dll
O3 - Toolbar: &Google - {2318C2B1-4965-11d4-9B18-009027A5CD4F} - c:\program files\google\googletoolbar1.dll
O4 - HKLM\..\Run: [avast!] D:\Avast\ashDisp.exe
O4 - HKLM\..\Run: [ashMaiSv] D:\Avast\ashmaisv.exe
O4 - HKLM\..\Run: [SmcService] D:\Sygate\smc.exe -startgui
O4 - HKLM\..\Run: [NvCplDaemon] RUNDLL32.EXE C:\WINNT\system32\NvCpl.dll,NvStartup
O4 - HKLM\..\Run: [QuickTime Task] "C:\WINNT\system32\qttask.exe" -atboottime
O4 - HKCU\..\Run: [Spyware Doctor] "D:\Spyware Doctor\Spyware Doctor\spydoctor.exe" /Q
O4 - HKCU\..\Run: [System Mechanic Startup Guard] "C:\Program Files\iolo\System Mechanic 5 Professional\StartupGuard.exe"
O4 - Global Startup: GoBack.lnk = D:\GoBack\GBTray.exe
O8 - Extra context menu item: &Google Search - res://C:\Program Files\Google\GoogleToolbar1.dll/cmsearch.html
O8 - Extra context menu item: &WordWeb... - res://C:\WINNT\system32\wweb32.dll/lookup.html
O8 - Extra context menu item: Backward Links - res://C:\Program Files\Google\GoogleToolbar1.dll/cmbacklinks.html
O8 - Extra context menu item: Cached Snapshot of Page - res://C:\Program Files\Google\GoogleToolbar1.dll/cmcache.html
O8 - Extra context menu item: Download Using &BitSpirit - D:\BitSpirit\BitSpirit\bsurl.htm
O8 - Extra context menu item: E&xport to Microsoft Excel - res://D:\MICROS~1\Office10\EXCEL.EXE/3000
O8 - Extra context menu item: Similar Pages - res://C:\Program Files\Google\GoogleToolbar1.dll/cmsimilar.html
O8 - Extra context menu item: Translate into English - res://C:\Program Files\Google\GoogleToolbar1.dll/cmtrans.html
O17 - HKLM\System\CCS\Services\Tcpip\..\{9AAFFBDD-4980-4E58-B1E1-86E6B8E80CAB}: NameServer = 202.96.134.133,61.144.56.100