How to increase disk space? [duplicate]
Remove old kernel packages (leave the latest one or two). I'd suggest something like Synaptic for this and you want to search the Installed section for linux-image and linux-headers.
The linux-image and linux-headers for each kernel version combines to about 200MB of disk space. On an old system that has gone through a lot of updates, this can easily give you back a few gigabytes of space.
A sudo apt-get clean will remove cached .debs that are sitting on disk from updates.