This will probably get long...
It's my basic process for manually cleaning a machine. When in doubt, back up the documents, and erase it and rebuild...
Otherwise, here it is:
Check Task Manager:
Control+Alt+Delete
On Win9x/winMe systems, it'll pop-up right away. On XP/win2k/win2k3, it'll be a button in the pop-up window. Pick it.
From the Task Manager, start doing a Google on any executables. SysTray and Explorer are standard windows stuff, so they're kosher. Googling the names of the other executables should teach you which ones are evil pretty quickly.
Make a list on paper of the evil ones. Then start killing those processes.
Don't reboot yet...
You'll be finding these evil things as .DLL and .EXE files. So be on the lookout. FYI, 'rundll' is the standard thing that loads DLL files. It's not evil, but when you see lines in the registry loading mystery DLLs, look up the DLLs, not RunDLL.
Next, do a RegEdit and look for "RunOnce" using the Find function (from the Edit menu in the menubar). Once you find RunOnce, look at the key right above it for "Run" (FYI, it's easier to find RunOnce, than Run) This is where lots of crap sets up shop. Once again, Google, and make a list of the evil ones. Right click the "Run" tree and back it up (for safety). Then start killing all the evil executables. Then do a search for "RunOnce" again, to find the other set of keys (there's 2 pairs in the registry).
Don't reboot yet...
Now, take that list and do a File Search (Start button, Search...) and look up all those evil files. Once you find them, you'll likely find them in odd directories of programs you didn't think you had. Or you'll find them in the Windows, Windows\System or Windows\System32 directory. Go delete the evil files or the directories to get the whole evil app. (NOTE: Do not be stupid and delete the Windows directory, etc...)
Don't reboot yet....
Last thing to hunt down is those pesky DLLs which are more invisible. They usually get launched via 'RunDLL' so you'll want to repeat the same set of hunting directions for the DLLs and EXEs.
Since you have a specific case of hunting down 'about_:blank' you've got a clue of what to find. When in doubt google for that specific string or if you've got an evil toolbar (there are a few) that won't die, google for the name of that toolbar that keeps showing up (check View/Toolbars to see the name, perhaps). Once you find it in Google, you should see a clue to what the files are. Do a file search and registry search for those files or directory and you should get lucky. That'll let you delete the file and registry instances.
It's tedious work, but I have used those steps to clean systems manually. When in doubt look stuff up in Google. And get a Google ToolBar (comes with Pop-Up blocker). And don't listen to pop-ups telling you to fix your PC. If your computer was really screwed up, it wouldn't be able to tell you...
Sometimes it is just easier to back up the data (over the wire is easiest) and then nuke and rebuild the box
Janx