Well all the following come to mind.
First, run the Disk Cleanup Utility, Start>All Programs>Accessories>System Tools>Disk Cleanup.
Then check if your disk needs to be Defragmented. Windows 7 schedules defragmentation of the system disk by default, but look and see if any disk needs defragmentation.
Check to see how much free space you have on the disk, about 15% of the total disk space is a good rule of thumb. Delete unnecessary file to free up space if needed.
Check your Startup Files, loading too many programs at startup can cause system problems. Make sure you load only the programs you really need. To check startups click Start then type msconfig and open the System Configuration dialog box. Select the Startup tab and uncheck the programs you don't need one at a time and reboot to see what affect removing that program has. Autoruns is much better utility to use than msconfig.
Check your Windows updates to see if you are current. If not update straight away.
Run a good Malware/Virus removal program. Update the definitions first then run just quick scans. If anything is found, run a full scan.
Uninstall unused programs. As a general rule, you should remove programs that you donít use on a regular basis. If you donít use it once in six months, for instance, you can probably get rid of it.
Check to see what Processes are running. Right-click an empty area of the taskbar and choose Task Manager. Use the Applications tab to see what full programs are running and the Processes tab to see whatís running behind the scenes. If you donít know what a process does, search Google for the process name you can find a ton of information. Process Explorer is a better utility than Task Manager for this.
Run a chldsk on the C: drive. In Windows explorer right click on the drive and select Properties. Then select Tools tab. Select Check now, check both check boxes, click start. Probably well have to reboot to run chkdsk since system disk is in use.
Clear your browser cache and remove excessive add-ons and toolbars.
Still running slow try running sfc /scannow in an elevated command prompt.
Read this post Slow Computer/browser? Check Here First; It May Not Be Malware
That should be enought to get you started.
Hope it helps.