If an installation fails, the downloaded
.apk might still be left in
/data/local (where the Playstore app downloads it to). In that case, a simple boot should clear it up.
As your device is rooted, you also could use a file manager to check that location, and manually delete the corresponding file.
One more hint: To avoid the "insufficient memory" / "insufficient storage" error (see: insufficient-memory, hover you mouse over the tag and select the "info" link for more details), which also might be responsible for failed installations, you will need at least 25MB free space on "internal storage" / "phone storage" (that is, this amount should be available when the installation has finished).