You could just lower the disk usage space for system restore.
System properties - system restore - settings
This would mean less space for restore points to be kept , so then the older ones would be deleted or overwritten when a new restore point is created.
EDIT: Found some more info, but do this at your own risk.
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WARNING: Information found in this section is for advanced users only. If you use this information without advanced knowledge of your operating system you can cause serious damage to your Windows installation.
All restore points are stored in a folder that starts with _restore in the System Volume Information folder found in the root of your individual partitions. This folder is used to store copies of your registry, files, configs, etc. The system volume information folder is only accessible to the System account by default. You can access this folder, though, by adding yourself to the security permissions of this account. Though its advised that you do not do so, I am sure that some of you will poke around in there anyway. XP Home users may not be able to add themselves to the security of this folder as XP home does not provide that level of security access.
There is a way, but not nearly tested enough, that you can delete individual restore points by going into the System Volume Information folder and delete the restore point folder that is associated with it. When in the _restore folder you will see other folders that start with RP and a number, such as RP9. These folders are your individual restore points. Compare the creation time of the folders to the time and date found in the System Restore Utility to determine which folder corresponds to the restore point you would like to delete. If you delete one of these folders and then go back into the system restore utility, you will notice that particular restore point no longer exists. One thing to keep in mind, which has not been tested, if you decide to delete a restore point in this manner, make sure you check the System Volume Information folders found on your other partitions to determine if that particular restore point also made folders there.
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I personally would just recommend deleting restore points using the disk cleanup utility and removing all but the most recent points, or just lowering your system restore disk space size to store less restore points.