Upgrading, but need to format old HDD to sell.

Yes, you can safely use your Win 7 Install CD to Delete any and all Partitions on that old HDD. Then you can stop the process with NO damage to anything. For most purposes, this is good enough.

You should be aware that someone who wants to put some effort into it could use data recovery software to get your old data from that HDD, even if you Delete its Partitions. But that won't be an accident. Anybody who merely tried to look at data on the drive would find it impossible. Anybody who Installs a new Windows of their own on it will wipe out all the old stuff, and then your old data will be almost impossible to recover. I'm just saying that someone who knows how to use data recovery software and wants to look at you old data could do it.

There are a couple of ways to prevent even that, but you may not want to bother. If you are really concerned, the first way is simply called a "Zero Fill" of the old HDD. This process writes zeroes to everywhere on the drive, thus over-writing all its old contents. After this process, only sophisticated expert systems could try to recover old data. A Zero Fill can be done with some free utilities from the company that made your old HDD. Or, you can download a software pack like DBAN and use it for the task.

The ultimate data destroyer, which I'm sure you do NOT need, is Military-grade data destruction. Such software writes a series of several data patterns (not just zeroes) to every part of the HDD. After that, even sophisticated tools can't figure out what the original data was.