It's not really 'memory' in how most would imply the word, but the simplest way you can avoid it getting full is by avoiding installing craptons of stuff or downloading things as that's more or less how you fill an HDD/SDD up.
Judging by what's in your screenshot of the D: drive though (not that either screenshot is really useful in this particular case as there are tons and tons of subfolders in each of those folders), and my experience with dell, D: is most likely a recovery drive where Dell keeps backups of certain things in the event of a system recovery through their tools. That one being mostly-full isn't necessarily too unusual as the system recovery "drive" is really just a partition on the drive.
As for C:, you can try using the
disk cleanup tool built into windows, to at least clean out things like temporary files, old windows update packages, amongst other things.
Alternatively also check your
Add/Remove Programs tool to see if there are any programs you may want uninstalled that you don't use (this might be more suited to someone a bit more tech-familiar however so you don't just go uninstalling tons of random stuff and screw up your windows).
More than likely the disk cleanup will clean out a lot of stuff, considering you're on vista and the Inspiron 1520 is a roughly 7 year old model, which it looks like your system may have literally been running since that period judging by some of the timestamps of your files, you likely have an incredible amount of temporary files and stuff if no one has ever bothered to run the cleanup tool or cleanout temp files before.
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