|1. try running a cleaner such as CCleaner. Pay attention during the install & make sure to uncheck the box for Yahoo Toolbar (it's spyware):|
2. install & run MalwareByte's Anti-Malware:
3. if that doesn't fix the problem, try running the System File Checker:
I believe you'll lose all updates, but since you're trying to install SP4 anyway, it shouldn't be a big deal.