Try defragmenting the hard drive. Try Firefox and see if it has the same performance issues as IE7. Check the virtual memory settings under Computer Properties, Advanced, Performance.
Sometimes, even you thought that you have already cleaned your system and removed your suspected virus infection, there might be still other malware programs that automatically execute every time you start your computer. There are many factors affecting the speed of the computer system, you also need to consider the hardware specifications of the computer.
Even if you believe that you have already cleaned any spyware or virus files in your computer system, it gets slow because there might be files duplicated and modified that are essential for a fast computer response.