This is my guess. Wait for Crunchie or someone knowledgable to confirm it/tell me what I've missed before proceeding. (I'm currently practising till one day I'll be as knowledgable as them. Unlikely but there we go!)
alt+ctrl+del and kill C:\Program Files\eMusic\eMusicClient.exe (the process).
Then fix the following:
F3 - REG:win.ini: load=????????????????????????
O2 - BHO: F1 Organizer Class - {00000EF1-0786-4633-87C6-1AA7A44296DA} - C:\WINDOWS\system32\ATPART~1.DLL
O2 - BHO: InstaFinder - {4E7BD74F-2B8D-469E-DCF7-F96DA086B434} - C:\WINDOWS\DOWNLO~1\instafin.dll
O2 - BHO: Search404 Class - {53C330D6-A4AB-419B-B45D-FD4411C1FEF4} - C:\Program Files\404Search\404Search.dll
O4 - HKLM\..\Run: [eMusicClient Systray] C:\Program Files\eMusic\eMusicClient.exe
O4 - Startup: PowerReg Scheduler V3.exe
O4 - Startup: Run Time.exe
O9 - Extra button: 50 FREE MP3s! - {686C970F-1D7D-4469-85D1-4B35763B56CC} - http://www.emusic.com/?fref=149031 (file missing)
Then boot in safe mode by presssing the f8 key during startup, and delete:
C:\WINDOWS\system32\ATPART~1.DLL
C:\WINDOWS\DOWNLO~1\instafin.dll
C:\Program Files\404Search\404Search.dll
Before doing this open Windows Explorer & Go to Tools > Folder Options. Click on the View tab and make sure that "Show hidden files and folders" is checked. Also uncheck "Hide protected operating system files" and untick "hide extensions for known file types" . Now click "Apply to all folders"
Click "Apply" then "OK"
[REVERSE THIS PROCESS AFTER YOUR CLEAN!!]