A fragmented hard drive can cause a system to operate sluggishly, especially when launching applications, as it takes longer for the system to access files that are spread out across the disk. Defragmenting the hard drive reorganizes the data so that files are stored in contiguous sections, which speeds up disk access times. Updating the OS may not directly address sluggish application launch times unless the issue is related to a specific bug that the update resolves. Running disk cleanup may help by removing temporary files, but it doesn't typically provide the performance improvement associated with defragmentation for this specific symptom. Adding more RAM can improve performance, but based on the given information, there's no indication that lack of RAM is the cause of the sluggishness.