Open up Hijack This in safe mode, check and remove these entries.

R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://www.gigasearch.biz/?134
R3 - URLSearchHook: GStartBHO Class - {EADD3112-0CF8-444b-AC0F-EBA38E004554} - C:\WINDOWS\Downloaded Program Files\giga32.dll
O2 - BHO: GStartBHO Class - {EADD3112-0CF8-444b-AC0F-EBA38E004554} - C:\WINDOWS\Downloaded Program Files\giga32.dll
O3 - Toolbar: GIGA Search - {D941BEA3-81E9-4033-8822-A733E2A91698} - C:\WINDOWS\Downloaded Program Files\giga32.dll

Then reboot normally and tell us the results. If it's fixed, you're done, if it's not, then attach the hijack this log again.