My current hard drive is a WD Green 5400 rpm. Its very slow. My pc specs are i5-4440, gtx 970 and 16gb ram. Even though having such a powerful setup my pc doesn't perform as expected and I think the hard drive is causing the problem. Apps load very slow. Games sometimes dont open at all. Should I upgrade to a new hard drive?
How old is this PC? 5400 RPM drives are uncommon in a desktop PC, and if it was used or off-hand then most definitely time for an upgrade to even an SSD if possible. If every other component is functioning completely fine, I would place the blame on a hard drive issue.
WD Green drives are great if you are looking for inexpensive storage capacity. However, they are not the hard drives to get if you want performance; in that case you want WD Black (7200 RPM). But keep in mind that all hard drives are slow when compared to a slow SSD. Even 10,000 RPM hard drives with SCSI interfaces are slow compared to a slow SSD.
If a game is not launching, then it is either a problem with Windows or the game did not install properly. There could also be background processes that are keeping your CPU busy which means it takes longer to load programs. Use Task Manager (right click the Windows status bar) to check what programs are keeping your CPU busy. It is possible you could have malware or a virus that is the cause so do a virus scan.
Also, check if your hard drive is fragmented. The more fragmented the hard drive is the longer it can take to launch programs. I usually defrag hard drives when the fragmentation is 10%+. Side note, never defrag a SSD, that will shorten it's lifespan.