According to this review there is no optical drive, (CD/DVD drive),http://www.naaptol.com/laptops/acer-aspire-3684nwxmi/P/779.html
The new harddrive is blank. No Operating System on it.
After a person installs a new harddrive, the basic procedure is to use the Restoration Disk, and when asked choose Full Install.
No optical drive.
[No Restoration Disk is NO problem. Contact HP Support.
In the US it is $17 for a 5 to 7 day ship, $24 for a 2 to 3 day ship.
Have more info for US, and other countries if you need it ]
This means you will go through some 'changes', if no optical drive.
More than I believe you may wish to take on.
(Don't want to overstep myself, or seem rude)
The next thing is there may be a harddrive limit.
Might be capped at 135GB, and not allow a 300, or 500GB harddrive.
If this holds true the harddrive is then partitioned;
Partitioned into 3 partitions, of 100GB each; or 4 partitions of 75GB each.
Partitioned into 4 partitions of 125GB each.
Fools BIOS into thinking there is just a 75GB, or 100GB, or 125GB harddrive; instead of a 300GB, or 500GB unit.
Yes. More headache if you may not be adept.
I suggest using an external harddrive instead.
Just my thoughts,