Hello Anitoff:
From the available literature it appears to have a 2.5 inch hard drive, so yes you can replace it with a larger hard drive. If I am looking at the right spec sheet this is a fairly old unit, that originally came with the Windows 98 second edition.

I'm not sure what operating system you are currently running, however the maximum hard drive size that can be read with Windows 98 second edition is 137 GB (I think), since I have limited experience with 98 second edition I am going to assume that there is a patch available to fix that.

I'm also going to assume that you really don't need a very large hard drive if you're living with 4 GB. I have included a links to two different Internet computer parts suppliers, so you can price hard drives.

If you are running Windows 98 second edition there may well be a patch available if you decide to run a drive larger than 137 GB, and since there is very little price difference between an 80 GB and 160 Gigabytes Drive that's probably the way I would go.

Additionally it will probably necessary to partition the hard drive, I would run at least two separate partitions anyway whether it had to be partitioned or not.

I have also included a link to the manufacturers website with the specification sheet I was looking at, so if I have been looking at the wrong spec sheet you will want to do further investigation.

http://www.4allmemory.com/index.cfm?fuseaction=search.memorySearch3&model_id=31672&product_id=238027&product_type=Other&p=393

http://www.mwave.com/mwave/Deepsearchdd.hmx?UID=&CID=

http://www.acersupport.com/notebook/html/tm515tx_specs.html

PS. If I am looking at the right spec sheet it is very important that you order an IDE hard drive, most of the newer units are SATA and will not work in your laptop.

however the maximum hard drive size that can be read with Windows 98 second edition is 137 GB (I think

you have to enter the BIOS, select a user defined option for your drive type, then enter 1023 cylinders, 16 heads, and 63 sectors. Format the first partition no larger than 8GB.

Thank you for your usefull advices.
Actually, I had an old 515T without HDD, but which once could begin to boot from W2K.I intended to use it to draw printed circuits boards, and 4GByte is more than enough for this purpose.But it does not boot anymore, for an unknown reason.
Either, I had to power it from a car battery, so, probably, I won't need an HDD anymore.T.Y.

That makes sense...Glad to be of help