I'm having a weird and frustrating problem with my laptop (Dell Latitude E5520 - i5 8GB RAM).
The system is very slow sometimes (for random periods of time), For example if I open extra tabs on a browser or playing audio on Winamp I'm experiencing a huge lag / freeze for few seconds or less, the hard drive light is on when experiencing the slowness.
After performing a lot of checks (resource monitor observation (doesn't seem to be the CPU / network or memory), dell diagnostics + thorough memory checks, drivers updates, registry clean with Registry Reviver, deep virus checks, windows hard drive error checks, deep scans with DiskCheckup and SeaTools and full defrag of both partitions) I noticed that the performance are increasing (less lag and almost no freezes) when performing the two Hard Drive checks (with DiskCheckup and SeaTools), It's like these tests are "helping" the HDD not to "suffocate" .
The problem began few days after adding another monitor (Dell 24'), it began all of a sudden although I eliminated the chance that its related to the monitor or its new driver after experiencing the same issues on Safe Mode with no additional monitor or any other external device connected.
My hard drive is Toshiba MK5061GS, Running on Win7
As a last ditch attempt before formatting, go through the OS instillation process and instead of formatting and making a new instillation tell it to repair the OS. You may have a corrupted system 32 file or picked something up that is hiding from scans.
When you boot the Windows CD it will have an option to repair at the start, skip that one it uses command and requires you to know exactly what is wrong.