Win 7's built in Resource Monitor lets you monitor which processes and applications are consuming most system resources. You can keep an eye on your cpu usage, memory usage, disk status and network status.
1) Click Start button, type resmon in Start Search and hit enter.
2) On the main window, the Overview tab will be pre-selected. Here you will get an overview of the resource consumption. CPU, Disk, Network, and Memory are all there.
3) When you click on the CPU tab, it shows you Real-Time processes, CPU usage and Maximum frequency. On the right side, you can see a graphical representation of the same. If you want more details, you can look for related handles and modules.
4) Similarly Memory tab shows the memory consumption process wise, Disk tab gives details of disk activity, and so on.
Once you've gone through each tab, identify the programs that are consuming maximum resources. If they are set to start at startup, disable them in the Startup tab of MSCONFIG. Windows processes such as Indexing and Search can be trimmed/disabled. For 3rd party apps, try tweaking them or using alternatives.