How to deactivate Creative Suite 5 serial number on dead computers?

CS5 is really old software.  Adobe no longer supports it.  The only thing they can help you with is an activation count reset.  That's assuming your current computer can support legacy software.  Many do not.

In the absence of an older computer that can support legacy 32-bit software, you may need to upgrade to modern apps.

MODERN OPTIONS:
Photoshop Elements or Premiere Elements (approx $100, no subscription needed).
https://www.adobe.com/products/photoshop-elements.html

Creative Cloud Photography Plan (approx $10/month for 12 months). Includes the following:

• Photoshop CC and Photoshop on iPad
• Camera Raw
• Lightroom on desktop, mobile and the web
• Lightroom Classic
• Portfolio website + hosting
• Adobe Express (free starter plan)
• Bridge
• 20 GB cloud storage (upgradable to 1 TB).
• Free upgrades for paid subscribers

https://www.adobe.com/creativecloud/photography.html

Premium Adobe Express ($10/month).

• Creative Cloud Express app (web and mobile)
• Premiere Rush (desktop and mobile)
• Photoshop Express (mobile)
• Spark Video (mobile)
• Spark Page (mobile)
• Adobe Portfolio + hosting
• 100 GB of cloud storage
• All premium templates and design assets including the collection of over 160 million royalty-free Adobe Stock photo collection.

https://helpx.adobe.com/express/using/express-overview.html

Creative Cloud All Apps (51 + apps & services) and Single App Plans
https://www.adobe.com/creativecloud/plans.html

Hope that helps.

you can check your account if you remember the adobe id used to purchase from adobe or regowith adobe: https://account.adobe.com

otherwise, requst an activation count reset from support. ask for nothing else related to cs5:

use a browser that allows popups and cookies and contact adobe support by clicking here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human

p.s you can also use phone (but chat is usually quicker), https://helpx.adobe.com/contact/phone.html

p.p.s. you can also use twitter to tweet @AdobeCare

p.p.p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true